feat(S2-b): connect durable MQTT sync to capture workflow

FieldLogger/App.xaml.cs

@@ -1,15 +1,18 @@
 using FieldLogger.Services;
+using FieldLogger.Services.Sync;
 
 namespace FieldLogger;
 
 public partial class App : Application
 {
 	private readonly DeviceConnectionManager _connectionManager;
+	private readonly IMqttSyncService _syncService;
 
-	public App(DeviceConnectionManager connectionManager)
+	public App(DeviceConnectionManager connectionManager, IMqttSyncService syncService)
 	{
 		InitializeComponent();
 		_connectionManager = connectionManager;
+		_syncService = syncService;
 
 		Console.WriteLine("===== FIELD LOGGER APP STARTING =====");
 		Console.WriteLine($"Console output is working! Time: {DateTime.Now:HH:mm:ss}");
@@ -24,6 +27,7 @@ public partial class App : Application
 		{
 			Console.WriteLine("App: Window closing, disconnecting devices...");
 			await DisconnectAllDevicesAsync();
+			await _syncService.StopAsync();
 		};
 
 		return window;
@@ -54,4 +58,4 @@ public partial class App : Application
 			Console.WriteLine($"App: Error disconnecting devices: {ex.Message}");
 		}
 	}
-}
+}

FieldLogger/FieldLogger.csproj

@@ -85,6 +85,10 @@
 		<PackageReference Include="SQLitePCLRaw.bundle_green" Version="2.1.10" />
 	</ItemGroup>
 
+	<ItemGroup>
+		<ProjectReference Include="..\src\FieldLogger.Sync\FieldLogger.Sync.csproj" />
+	</ItemGroup>
+
 	<!-- Native map control (Google Maps on Android, Apple Maps on iOS/macOS). Not available on Windows,
 	     where MapPage falls back to a Google Maps JavaScript WebView instead. -->
 	<ItemGroup Condition="$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)')) != 'windows'">

FieldLogger/MauiProgram.cs

@@ -36,7 +36,7 @@ public static class MauiProgram
 		builder.Services.AddSingleton<MaglinkService>();
 		builder.Services.AddSingleton<DeviceConnectionManager>();
 		builder.Services.AddSingleton<PointLogger>();
-		builder.Services.AddSingleton<IMqttSyncService, NullMqttSyncService>();
+		builder.Services.AddSingleton<IMqttSyncService, MqttSyncService>();
 
 		// View models
 		builder.Services.AddSingleton<HomeViewModel>();
@@ -60,6 +60,7 @@ public static class MauiProgram
 
 		// Instantiate the point logger so it listens for receiver packets from startup.
 		_ = app.Services.GetRequiredService<PointLogger>();
+		_ = app.Services.GetRequiredService<IMqttSyncService>().StartAsync();
 
 		return app;
 	}

FieldLogger/Models/LoggedPoint.cs

@@ -69,7 +69,13 @@ public sealed class LoggedPoint
     public string GpsRawSentence { get; set; } = "";
     public string GpsSerialNumber { get; set; } = "";
 
-    /// <summary>Reserved for MQTT sync.</summary>
+    /// <summary>Stable UUIDv7 used for MQTT retries and cloud idempotency.</summary>
+    [Indexed(Unique = true)]
+    public string? SyncPointId { get; set; }
+
+    /// <summary>Machine-readable terminal/configuration error; null while pending or after success.</summary>
+    public string? SyncError { get; set; }
+
     public bool Synced { get; set; }
 
     [Ignore]

FieldLogger/Services/Data/AppDatabase.cs

@@ -76,6 +76,23 @@ public sealed class AppDatabase
             .ToListAsync();
     }
 
+    public async Task<List<LoggedPoint>> GetUnsyncedPointsAsync()
+    {
+        var db = await Db;
+        return await db.Table<LoggedPoint>()
+            .Where(point => !point.Synced)
+            .OrderBy(point => point.TimestampUtc)
+            .ToListAsync();
+    }
+
+    public async Task<LoggedPoint?> GetPointBySyncIdAsync(string syncPointId)
+    {
+        var db = await Db;
+        return await db.Table<LoggedPoint>()
+            .Where(point => point.SyncPointId == syncPointId)
+            .FirstOrDefaultAsync();
+    }
+
     public async Task<int> GetPointCountAsync(int jobId)
     {
         var db = await Db;
@@ -87,4 +104,27 @@ public sealed class AppDatabase
         var db = await Db;
         await db.DeleteAsync<LoggedPoint>(pointId);
     }
+
+    public async Task UpdatePointAsync(LoggedPoint point)
+    {
+        var db = await Db;
+        await db.UpdateAsync(point);
+    }
+
+    public async Task MarkPointSyncedAsync(string syncPointId)
+    {
+        var db = await Db;
+        await db.ExecuteAsync(
+            "UPDATE points SET Synced = 1, SyncError = NULL WHERE SyncPointId = ?",
+            syncPointId);
+    }
+
+    public async Task MarkPointSyncErrorAsync(string syncPointId, string reasonCode)
+    {
+        var db = await Db;
+        await db.ExecuteAsync(
+            "UPDATE points SET Synced = 0, SyncError = ? WHERE SyncPointId = ?",
+            reasonCode,
+            syncPointId);
+    }
 }

FieldLogger/Services/PointLogger.cs

@@ -1,5 +1,6 @@
 using FieldLogger.Models;
 using FieldLogger.Services.Data;
+using FieldLogger.Services.Sync;
 using Microsoft.Extensions.Logging;
 
 namespace FieldLogger.Services;
@@ -14,6 +15,7 @@ public sealed class PointLogger
     private readonly MaglinkService _gps;
     private readonly AppDatabase _database;
     private readonly SettingsService _settings;
+    private readonly IMqttSyncService _sync;
     private readonly ILogger<PointLogger> _logger;
 
     /// <summary>Raised (on the UI thread) after a point is saved.</summary>
@@ -23,12 +25,13 @@ public sealed class PointLogger
     public event EventHandler? PacketIgnoredNoJob;
 
     public PointLogger(UmReceiverService locator, MaglinkService gps, AppDatabase database,
-        SettingsService settings, ILogger<PointLogger> logger)
+        SettingsService settings, IMqttSyncService sync, ILogger<PointLogger> logger)
     {
         _locator = locator;
         _gps = gps;
         _database = database;
         _settings = settings;
+        _sync = sync;
         _logger = logger;
 
         _locator.PacketReceived += OnPacketReceived;
@@ -53,10 +56,13 @@ public sealed class PointLogger
 
             var fix = _gps.FreshFix();
             var point = LoggedPoint.From(jobId.Value, packet, _locator.DeviceInfo, fix, _gps.DeviceInfo);
+            point.SyncPointId = Guid.CreateVersion7().ToString();
             await _database.AddPointAsync(point);
+            await _sync.PublishPointAsync(point);
 
-            _logger.LogInformation("Point {Id} saved to job {JobId} (gps valid: {GpsValid})",
+            _logger.LogInformation(
-                point.Id, jobId, point.GpsValid);
+                "Point {Id}/{SyncPointId} saved to job {JobId} (gps valid: {GpsValid}, sync error: {SyncError})",
+                point.Id, point.SyncPointId, jobId, point.GpsValid, point.SyncError);
             MainThread.BeginInvokeOnMainThread(() => PointSaved?.Invoke(this, point));
         }
         catch (Exception ex)

FieldLogger/Services/SettingsService.cs

@@ -11,6 +11,12 @@ public sealed class SettingsService
     private const string RtkNameKey = "device.rtk.name";
     private const string ActiveJobKey = "job.active.id";
     private const string MapsApiKeyKey = "maps.apikey";
+    private const string MqttEnabledKey = "mqtt.enabled";
+    private const string MqttHostKey = "mqtt.host";
+    private const string MqttPortKey = "mqtt.port";
+    private const string MqttOrgIdKey = "mqtt.org.id";
+    private const string MqttClientIdKey = "mqtt.client.id";
+    private const string MqttPasswordKey = "mqtt.password";
 
     public Guid? GetSavedDeviceId(DeviceKind kind)
     {
@@ -53,6 +59,52 @@ public sealed class SettingsService
         set => Preferences.Default.Set(MapsApiKeyKey, value);
     }
 
+    public bool MqttEnabled
+    {
+        get => Preferences.Default.Get(MqttEnabledKey, false);
+        set => Preferences.Default.Set(MqttEnabledKey, value);
+    }
+
+    public string MqttHost
+    {
+        get => Preferences.Default.Get(MqttHostKey, "dev.hub.umagul.net");
+        set => Preferences.Default.Set(MqttHostKey, value.Trim());
+    }
+
+    public int MqttPort
+    {
+        get => Preferences.Default.Get(MqttPortKey, 8884);
+        set => Preferences.Default.Set(MqttPortKey, value);
+    }
+
+    /// <summary>The interim MQTT username is the organization id.</summary>
+    public string MqttOrgId
+    {
+        get => Preferences.Default.Get(MqttOrgIdKey, string.Empty);
+        set => Preferences.Default.Set(MqttOrgIdKey, value.Trim());
+    }
+
+    public string MqttClientId
+    {
+        get
+        {
+            var existing = Preferences.Default.Get(MqttClientIdKey, string.Empty);
+            if (!string.IsNullOrWhiteSpace(existing)) return existing;
+            var created = $"fieldlogger-{Guid.NewGuid():N}";
+            Preferences.Default.Set(MqttClientIdKey, created);
+            return created;
+        }
+    }
+
+    public Task<string?> GetMqttPasswordAsync() => SecureStorage.Default.GetAsync(MqttPasswordKey);
+
+    public Task SetMqttPasswordAsync(string password) =>
+        string.IsNullOrWhiteSpace(password)
+            ? throw new ArgumentException("MQTT password cannot be empty.", nameof(password))
+            : SecureStorage.Default.SetAsync(MqttPasswordKey, password);
+
+    public void ClearMqttPassword() => SecureStorage.Default.Remove(MqttPasswordKey);
+
     private static string IdKey(DeviceKind kind) => kind == DeviceKind.Locator ? LocatorIdKey : RtkIdKey;
     private static string NameKey(DeviceKind kind) => kind == DeviceKind.Locator ? LocatorNameKey : RtkNameKey;
 }

FieldLogger/Services/Sync/IMqttSyncService.cs

@@ -3,25 +3,30 @@ using FieldLogger.Models;
 namespace FieldLogger.Services.Sync;
 
 /// <summary>
-/// Placeholder for the future MQTT synchronization layer:
+/// Durable app MQTT synchronization. Captures are persisted locally before this service queues
-/// - subscribe to receive jobs configured on the server
+/// them, and queue rows are released only by an application-level ACCEPTED/DUPLICATE ack.
-/// - publish logged points as they are captured
-/// - reconcile the Synced flags on <see cref="Job"/> and <see cref="LoggedPoint"/>
-/// Planned implementation: MQTTnet client against the configured broker.
 /// </summary>
 public interface IMqttSyncService
 {
     bool IsConnected { get; }
+    string Status { get; }
+    event EventHandler<string>? StatusChanged;
+    event EventHandler<PointSyncFailureEventArgs>? PointRejected;
+    Task StartAsync(CancellationToken cancellationToken = default);
+    Task StopAsync();
     Task ConnectAsync(CancellationToken cancellationToken = default);
     Task DisconnectAsync();
     Task PublishPointAsync(LoggedPoint point, CancellationToken cancellationToken = default);
 }
 
-/// <summary>No-op stand-in until the MQTT backend exists.</summary>
+public sealed class PointSyncFailureEventArgs : EventArgs
-public sealed class NullMqttSyncService : IMqttSyncService
 {
-    public bool IsConnected => false;
+    public LoggedPoint Point { get; }
-    public Task ConnectAsync(CancellationToken cancellationToken = default) => Task.CompletedTask;
+    public string ReasonCode { get; }
-    public Task DisconnectAsync() => Task.CompletedTask;
+
-    public Task PublishPointAsync(LoggedPoint point, CancellationToken cancellationToken = default) => Task.CompletedTask;
+    public PointSyncFailureEventArgs(LoggedPoint point, string reasonCode)
+    {
+        Point = point;
+        ReasonCode = reasonCode;
+    }
 }

FieldLogger/Services/Sync/MqttSyncService.cs

@@ -0,0 +1,395 @@
+using FieldLogger.Models;
+using FieldLogger.Services.Data;
+using Microsoft.Extensions.Logging;
+using SyncCore = FieldLogger.Sync;
+
+namespace FieldLogger.Services.Sync;
+
+/// <summary>
+/// Bridges the MAUI capture database to the workload-free durable MQTT engine. Network loss never
+/// deletes a capture: the local point and outbound queue are separate durable records, and an
+/// application acknowledgement is the only path that marks the local point synced.
+/// </summary>
+public sealed class MqttSyncService : IMqttSyncService
+{
+    private readonly SettingsService _settings;
+    private readonly AppDatabase _database;
+    private readonly ILogger<MqttSyncService> _logger;
+    private readonly SyncCore.SqliteOutboundStore _store;
+    private readonly SemaphoreSlim _connectionGate = new(1, 1);
+    private readonly SemaphoreSlim _pumpGate = new(1, 1);
+    private CancellationTokenSource? _lifetime;
+    private Task? _backgroundLoop;
+    private SyncCore.MqttnetTransport? _transport;
+    private SyncCore.MqttSyncEngine? _engine;
+    private bool _started;
+
+    public bool IsConnected => _transport?.IsConnected == true;
+    public string Status { get; private set; } = "Not started";
+    public event EventHandler<string>? StatusChanged;
+    public event EventHandler<PointSyncFailureEventArgs>? PointRejected;
+
+    public MqttSyncService(SettingsService settings, AppDatabase database, ILogger<MqttSyncService> logger)
+    {
+        _settings = settings;
+        _database = database;
+        _logger = logger;
+        _store = new SyncCore.SqliteOutboundStore(
+            Path.Combine(FileSystem.AppDataDirectory, "fieldlogger-sync.db3"));
+    }
+
+    public async Task StartAsync(CancellationToken cancellationToken = default)
+    {
+        if (_started) return;
+        _started = true;
+        await _store.InitAsync();
+        _lifetime = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
+        _backgroundLoop = RunAsync(_lifetime.Token);
+
+        if (_settings.MqttEnabled)
+        {
+            try
+            {
+                await ConnectAsync(cancellationToken);
+            }
+            catch (Exception ex)
+            {
+                _logger.LogWarning(ex, "Initial MQTT connection failed; durable retry loop remains active");
+                SetStatus($"Waiting to reconnect: {ex.Message}");
+            }
+        }
+        else
+        {
+            SetStatus("Sync disabled");
+        }
+    }
+
+    public async Task StopAsync()
+    {
+        _lifetime?.Cancel();
+        if (_backgroundLoop is not null)
+        {
+            try { await _backgroundLoop; }
+            catch (OperationCanceledException) { }
+        }
+        await DisconnectTransportAsync();
+        SetStatus("Stopped");
+    }
+
+    public async Task ConnectAsync(CancellationToken cancellationToken = default)
+    {
+        await ConnectCoreAsync(forceReconnect: true, cancellationToken);
+        await QueueUnsyncedAsync(cancellationToken);
+        await DrainAsync(cancellationToken);
+    }
+
+    public async Task DisconnectAsync()
+    {
+        await DisconnectTransportAsync();
+        SetStatus("Disconnected");
+    }
+
+    public async Task PublishPointAsync(LoggedPoint point, CancellationToken cancellationToken = default)
+    {
+        await _pumpGate.WaitAsync(cancellationToken);
+        try
+        {
+            await QueuePointAsync(point, cancellationToken);
+            if (IsConnected && _engine is not null)
+                await _engine.DrainOnceAsync(cancellationToken);
+        }
+        finally
+        {
+            _pumpGate.Release();
+        }
+    }
+
+    private async Task QueuePointAsync(LoggedPoint point, CancellationToken cancellationToken)
+    {
+        if (string.IsNullOrWhiteSpace(point.SyncPointId))
+        {
+            point.SyncPointId = Guid.CreateVersion7().ToString();
+            point.Synced = false;
+            await _database.UpdatePointAsync(point);
+        }
+
+        if (string.IsNullOrWhiteSpace(_settings.MqttOrgId))
+        {
+            await RecordFailureAsync(point, "SYNC_NOT_CONFIGURED");
+            return;
+        }
+
+        var job = await _database.GetJobAsync(point.JobId);
+        if (job is null)
+        {
+            await RecordFailureAsync(point, "LOCAL_JOB_NOT_FOUND");
+            return;
+        }
+
+        try
+        {
+            var record = ToSyncPoint(point);
+            var topic = $"ul/{_settings.MqttOrgId}/app/{_settings.MqttClientId}/log/points";
+            string? remoteJobId = string.IsNullOrWhiteSpace(job.RemoteId) ? null : job.RemoteId;
+            string? ticket = remoteJobId is null ? TicketFor(job) : null;
+            var outbound = SyncCore.OutboundMessage.FromPoint(
+                record,
+                topic,
+                DateTimeOffset.UtcNow,
+                jobId: remoteJobId,
+                ticket: ticket);
+            await _store.EnqueueAsync(outbound);
+            point.SyncError = null;
+            await _database.UpdatePointAsync(point);
+        }
+        catch (SyncCore.PointNotPublishableException ex)
+        {
+            await RecordFailureAsync(point, ex.ReasonCode);
+        }
+        catch (Exception ex)
+        {
+            _logger.LogError(ex, "Failed to enqueue point {PointId}", point.SyncPointId);
+            await RecordFailureAsync(point, "QUEUE_ERROR");
+        }
+        cancellationToken.ThrowIfCancellationRequested();
+    }
+
+    private async Task QueueUnsyncedAsync(CancellationToken cancellationToken)
+    {
+        foreach (var point in await _database.GetUnsyncedPointsAsync())
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+            // Permanent capture/server rejections require operator action, not an automatic loop.
+            if (point.SyncError is not null && point.SyncError is not "SYNC_NOT_CONFIGURED" and not "QUEUE_ERROR")
+                continue;
+            await QueuePointAsync(point, cancellationToken);
+        }
+    }
+
+    private async Task ConnectCoreAsync(bool forceReconnect, CancellationToken cancellationToken)
+    {
+        if (!_settings.MqttEnabled)
+            throw new InvalidOperationException("MQTT sync is disabled in Settings.");
+        if (string.IsNullOrWhiteSpace(_settings.MqttHost) || string.IsNullOrWhiteSpace(_settings.MqttOrgId))
+            throw new InvalidOperationException("MQTT host and organization id are required.");
+        string password = await _settings.GetMqttPasswordAsync()
+            ?? throw new InvalidOperationException("MQTT password is required.");
+
+        await _connectionGate.WaitAsync(cancellationToken);
+        try
+        {
+            if (IsConnected && !forceReconnect) return;
+            await DisconnectTransportAsync();
+
+            _transport = new SyncCore.MqttnetTransport(new SyncCore.MqttBrokerConfig
+            {
+                Host = _settings.MqttHost,
+                Port = _settings.MqttPort,
+                ClientId = _settings.MqttClientId,
+                Username = _settings.MqttOrgId,
+                Password = password,
+                UseTls = true,
+            });
+            _engine = new SyncCore.MqttSyncEngine(
+                _transport,
+                _store,
+                new SyncCore.SyncOptions
+                {
+                    OrgId = _settings.MqttOrgId,
+                    ClientId = _settings.MqttClientId,
+                });
+            _engine.PointAccepted += HandleAcceptedAsync;
+            _engine.PointRejected += HandleRejectedAsync;
+            await _engine.ConnectAsync(cancellationToken);
+            SetStatus("Connected");
+        }
+        finally
+        {
+            _connectionGate.Release();
+        }
+    }
+
+    private async Task RunAsync(CancellationToken cancellationToken)
+    {
+        using var timer = new PeriodicTimer(TimeSpan.FromSeconds(5));
+        while (await timer.WaitForNextTickAsync(cancellationToken))
+        {
+            if (!_settings.MqttEnabled)
+            {
+                SetStatus("Sync disabled");
+                continue;
+            }
+            try
+            {
+                if (!IsConnected)
+                {
+                    SetStatus("Connecting…");
+                    await ConnectCoreAsync(forceReconnect: false, cancellationToken);
+                    await QueueUnsyncedAsync(cancellationToken);
+                }
+                await DrainAsync(cancellationToken);
+            }
+            catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
+            {
+                throw;
+            }
+            catch (Exception ex)
+            {
+                _logger.LogWarning(ex, "MQTT sync iteration failed; queued data retained");
+                SetStatus($"Offline — retrying: {ex.Message}");
+            }
+        }
+    }
+
+    private async Task DrainAsync(CancellationToken cancellationToken)
+    {
+        if (_engine is null || !IsConnected) return;
+        await _pumpGate.WaitAsync(cancellationToken);
+        try
+        {
+            int published = await _engine.DrainOnceAsync(cancellationToken);
+            if (published > 0) SetStatus($"Connected — {published} point(s) awaiting ack");
+        }
+        finally
+        {
+            _pumpGate.Release();
+        }
+    }
+
+    private async Task HandleAcceptedAsync(string pointId)
+    {
+        await _database.MarkPointSyncedAsync(pointId);
+        SetStatus("Connected — synced");
+    }
+
+    private async Task HandleRejectedAsync(SyncCore.OutboundMessage message)
+    {
+        string reason = message.LastReason ?? "REJECTED";
+        await _database.MarkPointSyncErrorAsync(message.PointId, reason);
+        var point = await _database.GetPointBySyncIdAsync(message.PointId);
+        if (point is not null) PointRejected?.Invoke(this, new PointSyncFailureEventArgs(point, reason));
+        SetStatus($"Point rejected: {reason}");
+    }
+
+    private async Task RecordFailureAsync(LoggedPoint point, string reasonCode)
+    {
+        point.SyncError = reasonCode;
+        point.Synced = false;
+        await _database.UpdatePointAsync(point);
+        PointRejected?.Invoke(this, new PointSyncFailureEventArgs(point, reasonCode));
+        SetStatus($"Point not queued: {reasonCode}");
+    }
+
+    private async Task DisconnectTransportAsync()
+    {
+        if (_transport is null) return;
+        try { await _transport.DisposeAsync(); }
+        finally
+        {
+            _transport = null;
+            _engine = null;
+        }
+    }
+
+    private void SetStatus(string value)
+    {
+        if (Status == value) return;
+        Status = value;
+        StatusChanged?.Invoke(this, value);
+    }
+
+    private static string TicketFor(Job job)
+    {
+        string value = $"FL-{job.Id}-{job.Name}".Trim();
+        return value.Length <= 64 ? value : value[..64];
+    }
+
+    private static SyncCore.PointRecord ToSyncPoint(LoggedPoint point)
+    {
+        var timestamp = new DateTimeOffset(DateTime.SpecifyKind(point.TimestampUtc, DateTimeKind.Utc));
+        DateTimeOffset? positionEpoch = point.GpsUnixTimestamp > 0
+            ? DateTimeOffset.FromUnixTimeSeconds(point.GpsUnixTimestamp)
+            : null;
+        return new SyncCore.PointRecord
+        {
+            PointId = point.SyncPointId!,
+            Origin = SyncCore.PointOrigin.APP,
+            UploadPath = SyncCore.UploadPath.APP_MQTT,
+            CaptureTrigger = SyncCore.CaptureTrigger.LOCATOR_BUTTON,
+            CreatedAt = timestamp,
+            Position = point.GpsValid
+                ? new SyncCore.PositionGroup
+                {
+                    Lat = point.Latitude,
+                    Lon = point.Longitude,
+                    EllipsoidalHeight = point.Altitude,
+                    OrthometricHeight = point.AltitudeCorrected,
+                    PositionEpoch = positionEpoch,
+                }
+                : null,
+            Gnss = new SyncCore.GnssGroup
+            {
+                FixType = FixType(point.FixStatusEnum),
+                SatsUsed = point.SatellitesUsed,
+                Hdop = point.Hdop,
+                Hrms = point.Hrms,
+                Vrms = point.Vrms,
+                CorrectionAge = point.CorrectionAgeSeconds,
+                ReceiverSerial = point.GpsSerialNumber,
+                TiltAngle = point.TiltAngle,
+                Source = string.IsNullOrWhiteSpace(point.GpsSerialNumber) ? "PHONE" : "MAGLINK",
+            },
+            Locate = new SyncCore.LocateGroup
+            {
+                Depth = point.DepthMeters,
+                DepthUnits = point.DepthMeters is null ? null : "m",
+                SignalCurrent = point.CurrentMilliamps,
+                SignalStrength = point.Signal,
+                Frequency = point.Frequency,
+                Gain = point.GainDb,
+                LocateMode = LocateMode(point),
+                PhaseDegrees = point.LdPhase,
+                CompassDegrees = point.CompassAngle,
+                LocatorModel = point.LocatorModel,
+                LocatorSerial = point.LocatorSerialNumber,
+                TelemetryEpoch = timestamp,
+            },
+            Attributes = new SyncCore.AttributesGroup { UtilityType = UtilityType(point.UtilityEnum) },
+            Quality = new SyncCore.QualityGroup
+            {
+                QualityFlag = point.GpsValid && point.Hrms > 0 && point.Hrms <= 0.10
+                    ? "IN_SPEC"
+                    : "OUT_OF_SPEC",
+            },
+        };
+    }
+
+    private static string FixType(GnssFixStatus status) => status switch
+    {
+        GnssFixStatus.Single => "AUTONOMOUS",
+        GnssFixStatus.Dgps => "DGPS",
+        GnssFixStatus.RtkFloat => "FLOAT",
+        GnssFixStatus.RtkFixed => "FIXED",
+        _ => "NO_FIX",
+    };
+
+    private static string UtilityType(UmUtility utility) => utility switch
+    {
+        UmUtility.Gas => "GAS",
+        UmUtility.Power => "ELECTRIC",
+        UmUtility.Communications => "TELECOM",
+        UmUtility.Water => "WATER",
+        UmUtility.Sewer => "SEWER",
+        UmUtility.Fiber => "FIBER",
+        _ => "UNKNOWN",
+    };
+
+    private static string LocateMode(LoggedPoint point) => point.FreqType == (int)UmFreqType.Sonde
+        ? "SONDE"
+        : (UmMode)point.Mode switch
+        {
+            UmMode.Null => "NULL",
+            UmMode.Omni or UmMode.TwinOmni => "BROAD_PEAK",
+            _ => "PEAK",
+        };
+}

FieldLogger/ViewModels/HomeViewModel.cs

@@ -4,6 +4,7 @@ using CommunityToolkit.Mvvm.Input;
 using FieldLogger.Models;
 using FieldLogger.Services;
 using FieldLogger.Services.Data;
+using FieldLogger.Services.Sync;
 
 namespace FieldLogger.ViewModels;
 
@@ -14,6 +15,7 @@ public sealed partial class HomeViewModel : ObservableObject
     private readonly PointLogger _pointLogger;
     private readonly AppDatabase _database;
     private readonly SettingsService _settings;
+    private readonly IMqttSyncService _sync;
 
     public DeviceConnectionManager Manager => _manager;
 
@@ -35,20 +37,27 @@ public sealed partial class HomeViewModel : ObservableObject
     [ObservableProperty]
     private string _fixAccuracy = "";
 
+    [ObservableProperty]
+    private string _syncStatus = "";
+
     public ObservableCollection<LoggedPoint> RecentPoints { get; } = new();
 
     public HomeViewModel(DeviceConnectionManager manager, MaglinkService gps,
-        PointLogger pointLogger, AppDatabase database, SettingsService settings)
+        PointLogger pointLogger, AppDatabase database, SettingsService settings, IMqttSyncService sync)
     {
         _manager = manager;
         _gps = gps;
         _pointLogger = pointLogger;
         _database = database;
         _settings = settings;
+        _sync = sync;
+        SyncStatus = sync.Status;
 
         _gps.FixReceived += OnFixReceived;
         _pointLogger.PointSaved += OnPointSaved;
         _pointLogger.PacketIgnoredNoJob += OnPacketIgnoredNoJob;
+        _sync.StatusChanged += OnSyncStatusChanged;
+        _sync.PointRejected += OnPointRejected;
     }
 
     /// <summary>Called from the page's OnAppearing.</summary>
@@ -146,4 +155,14 @@ public sealed partial class HomeViewModel : ObservableObject
             "A point was logged on the receiver, but no job is active so it was not saved. Create or select a job first.",
             "OK");
     }
+
+    private void OnSyncStatusChanged(object? sender, string status) =>
+        MainThread.BeginInvokeOnMainThread(() => SyncStatus = status);
+
+    private void OnPointRejected(object? sender, PointSyncFailureEventArgs e) =>
+        MainThread.BeginInvokeOnMainThread(async () =>
+            await Shell.Current.DisplayAlert(
+                "Point Saved Locally — Sync Needs Attention",
+                $"Point #{e.Point.Id} remains on this device and was not uploaded. Reason: {e.ReasonCode}.",
+                "OK"));
 }

FieldLogger/ViewModels/SettingsViewModel.cs

@@ -2,6 +2,7 @@ using CommunityToolkit.Mvvm.ComponentModel;
 using CommunityToolkit.Mvvm.Input;
 using FieldLogger.Models;
 using FieldLogger.Services;
+using FieldLogger.Services.Sync;
 
 namespace FieldLogger.ViewModels;
 
@@ -9,23 +10,104 @@ public sealed partial class SettingsViewModel : ObservableObject
 {
     private readonly DeviceConnectionManager _manager;
     private readonly SettingsService _settings;
+    private readonly IMqttSyncService _sync;
 
     public DeviceConnectionManager Manager => _manager;
 
     [ObservableProperty]
     private string _mapsApiKey = "";
 
+    [ObservableProperty]
+    private bool _mqttEnabled;
+
+    [ObservableProperty]
+    private string _mqttHost = "";
+
+    [ObservableProperty]
+    private string _mqttPort = "8884";
+
+    [ObservableProperty]
+    private string _mqttOrgId = "";
+
+    [ObservableProperty]
+    private string _mqttPassword = "";
+
+    [ObservableProperty]
+    private string _syncStatus = "";
+
+    [ObservableProperty]
+    private bool _isSavingSync;
+
+    public string MqttClientId => _settings.MqttClientId;
+
     public string AppVersion => AppInfo.Current.VersionString;
 
-    public SettingsViewModel(DeviceConnectionManager manager, SettingsService settings)
+    public SettingsViewModel(DeviceConnectionManager manager, SettingsService settings, IMqttSyncService sync)
     {
         _manager = manager;
         _settings = settings;
+        _sync = sync;
         MapsApiKey = settings.GoogleMapsApiKey;
+        MqttEnabled = settings.MqttEnabled;
+        MqttHost = settings.MqttHost;
+        MqttPort = settings.MqttPort.ToString();
+        MqttOrgId = settings.MqttOrgId;
+        SyncStatus = sync.Status;
+        _sync.StatusChanged += OnSyncStatusChanged;
     }
 
     partial void OnMapsApiKeyChanged(string value) => _settings.GoogleMapsApiKey = value.Trim();
 
+    [RelayCommand]
+    private async Task SaveSyncAsync()
+    {
+        if (!int.TryParse(MqttPort, out var port) || port is < 1 or > 65535)
+        {
+            await Shell.Current.DisplayAlert("Invalid MQTT Port", "Enter a port between 1 and 65535.", "OK");
+            return;
+        }
+        if (MqttEnabled && (string.IsNullOrWhiteSpace(MqttHost) || string.IsNullOrWhiteSpace(MqttOrgId)))
+        {
+            await Shell.Current.DisplayAlert("Incomplete Sync Settings", "Host and organization id are required.", "OK");
+            return;
+        }
+
+        IsSavingSync = true;
+        try
+        {
+            _settings.MqttHost = MqttHost;
+            _settings.MqttPort = port;
+            _settings.MqttOrgId = MqttOrgId;
+            _settings.MqttEnabled = MqttEnabled;
+            if (!string.IsNullOrWhiteSpace(MqttPassword))
+            {
+                await _settings.SetMqttPasswordAsync(MqttPassword);
+                MqttPassword = "";
+            }
+
+            if (MqttEnabled)
+            {
+                await _sync.ConnectAsync();
+                await Shell.Current.DisplayAlert("Sync Connected", "The durable MQTT queue is connected.", "OK");
+            }
+            else
+            {
+                await _sync.DisconnectAsync();
+            }
+        }
+        catch (Exception ex)
+        {
+            await Shell.Current.DisplayAlert("Sync Connection Failed", ex.Message, "OK");
+        }
+        finally
+        {
+            IsSavingSync = false;
+        }
+    }
+
+    private void OnSyncStatusChanged(object? sender, string status) =>
+        MainThread.BeginInvokeOnMainThread(() => SyncStatus = status);
+
     [RelayCommand]
     private Task ChangeLocatorAsync() => Shell.Current.GoToAsync($"devicescan?kind={DeviceKind.Locator}");
 

FieldLogger/Views/HomePage.xaml

@@ -63,6 +63,7 @@
                     </Grid>
                     <Label Text="Press the log button on the receiver to capture a point."
                            FontSize="12" TextColor="Gray" IsVisible="{Binding HasActiveJob}" />
+                    <Label Text="{Binding SyncStatus}" FontSize="11" TextColor="DodgerBlue" />
                 </VerticalStackLayout>
             </Border>
 

FieldLogger/Views/SettingsPage.xaml

@@ -52,8 +52,19 @@
 
             <Label Text="Sync" FontAttributes="Bold" FontSize="16" Margin="0,12,0,0" />
             <Border StrokeThickness="0" Background="{AppThemeBinding Light=#F2F2F7, Dark=#1C1C1E}" StrokeShape="RoundRectangle 12" Padding="12">
-                <Label Text="MQTT job sync is planned. Data is currently stored locally on this device."
+                <VerticalStackLayout Spacing="8">
-                       FontSize="12" TextColor="Gray" />
+                    <Grid ColumnDefinitions="*,Auto">
+                        <Label Text="Durable cloud sync" VerticalOptions="Center" />
+                        <Switch Grid.Column="1" IsToggled="{Binding MqttEnabled}" />
+                    </Grid>
+                    <Entry Text="{Binding MqttHost}" Placeholder="dev.hub.umagul.net" />
+                    <Entry Text="{Binding MqttPort}" Placeholder="8884" Keyboard="Numeric" />
+                    <Entry Text="{Binding MqttOrgId}" Placeholder="Organization ID / MQTT username" />
+                    <Entry Text="{Binding MqttPassword}" Placeholder="New MQTT password (stored securely)" IsPassword="True" />
+                    <Label Text="{Binding MqttClientId, StringFormat='Client: {0}'}" FontSize="10" TextColor="Gray" />
+                    <Label Text="{Binding SyncStatus}" FontSize="12" TextColor="DodgerBlue" />
+                    <Button Text="Save &amp; Connect" Command="{Binding SaveSyncCommand}" />
+                </VerticalStackLayout>
             </Border>
 
             <Label Text="{Binding AppVersion, StringFormat='Field Logger v{0}'}"

src/FieldLogger.Sync/BackoffPolicy.cs

@@ -0,0 +1,25 @@
+namespace FieldLogger.Sync;
+
+/// <summary>Exponential retry backoff with full jitter and a hard cap.</summary>
+public sealed class BackoffPolicy
+{
+    private readonly TimeSpan _base;
+    private readonly TimeSpan _cap;
+    private readonly Random _random;
+
+    public BackoffPolicy(TimeSpan baseDelay, TimeSpan cap, Random? random = null)
+    {
+        if (baseDelay <= TimeSpan.Zero) throw new ArgumentOutOfRangeException(nameof(baseDelay));
+        if (cap < baseDelay) throw new ArgumentOutOfRangeException(nameof(cap));
+        _base = baseDelay;
+        _cap = cap;
+        _random = random ?? Random.Shared;
+    }
+
+    public TimeSpan NextDelay(int attempt)
+    {
+        int exponent = Math.Min(Math.Max(1, attempt) - 1, 30);
+        double ceilingMs = Math.Min(_cap.TotalMilliseconds, _base.TotalMilliseconds * Math.Pow(2, exponent));
+        return TimeSpan.FromMilliseconds(_random.NextDouble() * ceilingMs);
+    }
+}

src/FieldLogger.Sync/FieldLogger.Sync.csproj

@@ -0,0 +1,27 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <!--
+    Cloud-sync core for Field Logger: the durable outbound queue, the Point wire mapping,
+    and the MQTTS publish/ack engine (SRS-SYN-2/5/7, §3.4.2, telemetry-schema.md).
+    Deliberately a plain net9.0 library (NO MAUI head) so the queue-durability, ack-release,
+    and exactly-once behaviour run headless in the QA gate without the Android/iOS workloads.
+    The MAUI app references this and supplies a DB path + broker config; tests supply a fake
+    transport and a temp-file store.
+  -->
+
+  <PropertyGroup>
+    <TargetFramework>net9.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <LangVersion>latest</LangVersion>
+    <RootNamespace>FieldLogger.Sync</RootNamespace>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <!-- Real broker transport. Tests do not touch this (they use a fake IMqttTransport). -->
+    <PackageReference Include="MQTTnet" Version="4.3.7.1207" />
+    <PackageReference Include="sqlite-net-pcl" Version="1.9.172" />
+    <PackageReference Include="SQLitePCLRaw.bundle_green" Version="2.1.10" />
+  </ItemGroup>
+
+</Project>

src/FieldLogger.Sync/IMqttTransport.cs

@@ -0,0 +1,56 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace FieldLogger.Sync;
+
+/// <summary>
+/// Minimal MQTTS transport the sync engine drives. Abstracted so the engine's
+/// durability/ack/backoff behaviour is tested headless with a fake, while the app uses the
+/// MQTTnet-backed implementation against the real broker (§3.4.2).
+/// </summary>
+public interface IMqttTransport
+{
+    bool IsConnected { get; }
+
+    /// <summary>Connect (MQTTS/TLS) and subscribe to the ack topic. Throws on failure.</summary>
+    Task ConnectAsync(string ackTopic, CancellationToken ct = default);
+
+    Task DisconnectAsync();
+
+    /// <summary>Publish a payload at QoS 1 (durable). Completes on broker PUBACK; throws on failure.</summary>
+    Task PublishAsync(string topic, byte[] payload, string schemaVersion, CancellationToken ct = default);
+
+    /// <summary>Raised when an application-level ack payload arrives on the ack topic.</summary>
+    event Func<AckBatch, Task>? AckReceived;
+}
+
+/// <summary>
+/// Application-level ack payload on ul/{orgId}/app/{clientId}/ack (SRS §3.4.2 "…/ack:
+/// UUIDs accepted/rejected + reason"). Distinct from the broker PUBACK — the queue releases a
+/// record only on the application ack referencing its pointId.
+/// </summary>
+public sealed record AckBatch
+{
+    [JsonPropertyName("schemaVersion")] public string SchemaVersion { get; init; } = "1";
+    [JsonPropertyName("results")] public List<AckItem> Results { get; init; } = new();
+
+    private static readonly JsonSerializerOptions Opts = new()
+    {
+        Converters = { new JsonStringEnumConverter() },
+        PropertyNameCaseInsensitive = true,
+    };
+
+    public static AckBatch FromJsonUtf8(ReadOnlySpan<byte> utf8) =>
+        JsonSerializer.Deserialize<AckBatch>(utf8, Opts) ?? throw new JsonException("null AckBatch");
+    public byte[] ToJsonUtf8() => JsonSerializer.SerializeToUtf8Bytes(this, Opts);
+}
+
+public enum AckOutcome { ACCEPTED, DUPLICATE, REJECTED }
+
+public sealed record AckItem
+{
+    [JsonPropertyName("pointId")] public required string PointId { get; init; }
+    [JsonPropertyName("outcome")] public AckOutcome Outcome { get; init; }
+    /// <summary>Machine code when REJECTED (api.yaml Error catalog); null on ACCEPTED.</summary>
+    [JsonPropertyName("reasonCode")] public string? ReasonCode { get; init; }
+}

src/FieldLogger.Sync/MqttSyncEngine.cs

@@ -0,0 +1,134 @@
+namespace FieldLogger.Sync;
+
+/// <summary>Tunables for the publish/retry/ack loop.</summary>
+public sealed record SyncOptions
+{
+    /// <summary>ul/{orgId}/app/{clientId} — the app's own namespace root. The engine only ever
+    /// publishes/subscribes under this prefix (namespace confinement; SRS §3.4.2 / §10.2).</summary>
+    public required string OrgId { get; init; }
+    public required string ClientId { get; init; }
+
+    public string PointsTopic => $"ul/{OrgId}/app/{ClientId}/log/points";
+    public string AckTopic => $"ul/{OrgId}/app/{ClientId}/ack";
+
+    /// <summary>How long to wait for the application-level ack before republishing (idempotent
+    /// via UUID; cloud dedups). Not a failure — release still requires the ack.</summary>
+    public TimeSpan AckWindow { get; init; } = TimeSpan.FromSeconds(10);
+    /// <summary>Backoff on broker/publish failure: base, doubled per attempt, capped.</summary>
+    public TimeSpan BackoffBase { get; init; } = TimeSpan.FromSeconds(1);
+    public TimeSpan BackoffCap { get; init; } = TimeSpan.FromSeconds(60);
+    public int DrainBatch { get; init; } = 50;
+
+    /// <summary>Reason codes that make a REJECTED ack terminal (drop from queue + surface for
+    /// LOG-7) rather than retryable. Everything else is retried with backoff. Default: schema /
+    /// validation / authorization failures — resending won't help.</summary>
+    public IReadOnlySet<string> TerminalRejectCodes { get; init; } = new HashSet<string>(StringComparer.OrdinalIgnoreCase)
+    {
+        "VALIDATION_ERROR", "POINT_ID_NOT_UUIDV7", "ENVELOPE_VALIDATION_ERROR",
+        "UNKNOWN_JOB_OR_WRONG_ORG", "SCHEMA_INVALID", "FORBIDDEN", "UNAUTHENTICATED",
+        "NOT_FOUND", "PAYLOAD_TOO_LARGE",
+    };
+}
+
+/// <summary>
+/// Drives the durable outbound queue to the broker and releases records only on the cloud's
+/// application-level ack (SRS-SYN-2/7, §3.4.2). Publish is idempotent via the UUIDv7 pointId,
+/// so an unacked record is safely republished after the ack window and the cloud de-duplicates.
+/// Broker loss never loses a capture — the record stays durably queued (SRS-SYN-1). Transport-
+/// and store-agnostic so it runs headless in tests.
+/// </summary>
+public sealed class MqttSyncEngine
+{
+    private readonly IMqttTransport _transport;
+    private readonly IOutboundStore _store;
+    private readonly SyncOptions _opts;
+    private readonly Func<DateTimeOffset> _now;
+    private readonly BackoffPolicy _backoff;
+
+    /// <summary>Raised when the cloud terminally rejects a record (LOG-7 surface: never silent).</summary>
+    public event Func<OutboundMessage, Task>? PointRejected;
+    /// <summary>Raised when a record is accepted and released from the queue.</summary>
+    public event Func<string, Task>? PointAccepted;
+
+    public MqttSyncEngine(IMqttTransport transport, IOutboundStore store, SyncOptions opts,
+        Func<DateTimeOffset>? now = null, BackoffPolicy? backoff = null)
+    {
+        _transport = transport;
+        _store = store;
+        _opts = opts;
+        _now = now ?? (() => DateTimeOffset.UtcNow);
+        _backoff = backoff ?? new BackoffPolicy(opts.BackoffBase, opts.BackoffCap);
+        _transport.AckReceived += ApplyAckAsync;
+    }
+
+    /// <summary>Connect + subscribe to the ack topic. Safe to call when a broker is reachable.</summary>
+    public Task ConnectAsync(CancellationToken ct = default) => _transport.ConnectAsync(_opts.AckTopic, ct);
+
+    /// <summary>
+    /// Publish every ready record once. Returns the number published this pass. On publish
+    /// failure (e.g. broker down) the record is rescheduled with backoff and kept — no capture
+    /// is lost. Records published successfully stay queued until their app-level ack arrives.
+    /// </summary>
+    public async Task<int> DrainOnceAsync(CancellationToken ct = default)
+    {
+        if (!_transport.IsConnected)
+            return 0; // broker unreachable → leave everything durably queued (SRS-SYN-1)
+
+        var ready = await _store.DequeueReadyAsync(_now(), _opts.DrainBatch);
+        int published = 0;
+        foreach (var m in ready)
+        {
+            ct.ThrowIfCancellationRequested();
+            try
+            {
+                await _transport.PublishAsync(m.Topic, System.Text.Encoding.UTF8.GetBytes(m.PayloadJson),
+                    m.SchemaVersion, ct);
+                // Published (broker PUBACK). Do NOT release — wait for the application ack.
+                // Reschedule a republish after the ack window in case the ack is lost.
+                await _store.TouchAwaitingAckAsync(m.PointId, _now() + _opts.AckWindow);
+                published++;
+            }
+            catch (Exception ex)
+            {
+                await _store.RescheduleAsync(m.PointId, _now() + _backoff.NextDelay(m.AttemptCount + 1), ex.Message);
+            }
+        }
+        return published;
+    }
+
+    private async Task ApplyAckAsync(AckBatch batch)
+    {
+        foreach (var item in batch.Results)
+        {
+            if (item.Outcome is AckOutcome.ACCEPTED or AckOutcome.DUPLICATE)
+            {
+                await _store.ReleaseAckedAsync(item.PointId);   // release only on ACCEPTED
+                if (PointAccepted is { } acceptedHandlers)
+                {
+                    foreach (Func<string, Task> handler in acceptedHandlers.GetInvocationList())
+                        await handler(item.PointId);
+                }
+            }
+            else // REJECTED
+            {
+                var code = item.ReasonCode ?? "REJECTED";
+                if (_opts.TerminalRejectCodes.Contains(code))
+                {
+                    await _store.MarkRejectedAsync(item.PointId, code);   // LOG-7: surface, never silent
+                    var rejected = (await _store.GetRejectedAsync()).FirstOrDefault(r => r.PointId == item.PointId);
+                    if (rejected is not null && PointRejected is { } rejectedHandlers)
+                    {
+                        foreach (Func<OutboundMessage, Task> handler in rejectedHandlers.GetInvocationList())
+                            await handler(rejected);
+                    }
+                }
+                else
+                {
+                    // Retryable rejection — reschedule immediately-ish with backoff.
+                    await _store.RescheduleAsync(item.PointId, _now() + _opts.BackoffBase, code);
+                }
+            }
+        }
+    }
+
+}

src/FieldLogger.Sync/MqttnetTransport.cs

@@ -0,0 +1,112 @@
+using System.Text;
+using MQTTnet;
+using MQTTnet.Client;
+using MQTTnet.Protocol;
+
+namespace FieldLogger.Sync;
+
+/// <summary>Interim app→broker connection config (SRS §3.4.2). MQTTS/TLS only; namespace-scoped
+/// credential. OIDC-derived tokens (SRS-SYN-3) are deferred to the auth sprint — see decisions.md.
+/// Reviewed by `security` (S2-sec).</summary>
+public sealed record MqttBrokerConfig
+{
+    public required string Host { get; init; }
+    public int Port { get; init; } = 8884;
+    public bool UseTls { get; init; } = true;
+    public required string ClientId { get; init; }
+    /// <summary>Interim credential (username = orgId, password = scoped per-org secret).
+    /// Supplied at runtime from secure storage / config — never hard-coded.</summary>
+    public string? Username { get; init; }
+    public string? Password { get; init; }
+    public TimeSpan SessionExpiry { get; init; } = TimeSpan.FromHours(24);
+}
+
+/// <summary>
+/// MQTTnet-backed <see cref="IMqttTransport"/> against the real UlHub broker. QoS 1 + persistent
+/// session so durable records survive reconnect; TLS mandatory. Ack payloads on the subscribed
+/// ack topic are surfaced via <see cref="AckReceived"/>. Not exercised by the headless tests
+/// (they use a fake transport) — this is the production path.
+/// </summary>
+public sealed class MqttnetTransport : IMqttTransport, IAsyncDisposable
+{
+    private readonly MqttBrokerConfig _config;
+    private readonly IMqttClient _client;
+    private string? _ackTopic;
+
+    public event Func<AckBatch, Task>? AckReceived;
+    public bool IsConnected => _client.IsConnected;
+
+    public MqttnetTransport(MqttBrokerConfig config)
+    {
+        _config = config;
+        _client = new MqttFactory().CreateMqttClient();
+        _client.ApplicationMessageReceivedAsync += OnMessageAsync;
+    }
+
+    public async Task ConnectAsync(string ackTopic, CancellationToken ct = default)
+    {
+        if (!_config.UseTls)
+            throw new InvalidOperationException("The app MQTT transport requires TLS.");
+        if (string.IsNullOrWhiteSpace(_config.Username) || string.IsNullOrWhiteSpace(_config.Password))
+            throw new InvalidOperationException("A scoped app MQTT username and password are required.");
+
+        _ackTopic = ackTopic;
+
+        var options = new MqttClientOptionsBuilder()
+            .WithTcpServer(_config.Host, _config.Port)
+            .WithClientId(_config.ClientId)
+            .WithProtocolVersion(MQTTnet.Formatter.MqttProtocolVersion.V500)
+            .WithCleanSession(false)                                  // persistent session (durable)
+            .WithSessionExpiryInterval((uint)_config.SessionExpiry.TotalSeconds)
+            .WithTlsOptions(o => o.UseTls(_config.UseTls))
+            .WithCredentials(_config.Username, _config.Password)
+            .Build();
+
+        await _client.ConnectAsync(options, ct);
+        await _client.SubscribeAsync(ackTopic, MqttQualityOfServiceLevel.AtLeastOnce, ct);
+    }
+
+    public Task DisconnectAsync() =>
+        _client.IsConnected ? _client.DisconnectAsync() : Task.CompletedTask;
+
+    public async Task PublishAsync(string topic, byte[] payload, string schemaVersion, CancellationToken ct = default)
+    {
+        var msg = new MqttApplicationMessageBuilder()
+            .WithTopic(topic)
+            .WithPayload(payload)
+            .WithQualityOfServiceLevel(MqttQualityOfServiceLevel.AtLeastOnce)   // QoS 1 (durable)
+            .WithContentType("application/json")
+            .WithUserProperty("schemaVersion", schemaVersion)
+            .Build();
+
+        var result = await _client.PublishAsync(msg, ct);
+        if (!result.IsSuccess)
+            throw new InvalidOperationException($"publish rejected: {result.ReasonCode}");
+    }
+
+    private async Task OnMessageAsync(MqttApplicationMessageReceivedEventArgs e)
+    {
+        if (e.ApplicationMessage.Topic == _ackTopic)
+        {
+            try
+            {
+                var batch = AckBatch.FromJsonUtf8(e.ApplicationMessage.PayloadSegment);
+                if (AckReceived is { } handlers)
+                {
+                    foreach (Func<AckBatch, Task> handler in handlers.GetInvocationList())
+                        await handler(batch);
+                }
+            }
+            catch
+            {
+                // Malformed ack payload — ignore; the record stays queued and is republished.
+            }
+        }
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        await DisconnectAsync();
+        _client.Dispose();
+    }
+}

src/FieldLogger.Sync/OutboundStore.cs

@@ -0,0 +1,159 @@
+using SQLite;
+
+namespace FieldLogger.Sync;
+
+/// <summary>Lifecycle of a queued outbound record.</summary>
+public enum OutboundStatus
+{
+    /// <summary>Ready to publish.</summary>
+    Pending = 0,
+    /// <summary>Published and waiting for an application acknowledgement.</summary>
+    InFlight = 1,
+    /// <summary>Terminally rejected by the cloud (schema/validation/authz) — not retried; surfaced for LOG-7.</summary>
+    RejectedTerminal = 2,
+}
+
+/// <summary>
+/// A durably-queued outbound point. Survives app restart (persisted in SQLite) and is
+/// released only when the cloud application-level ack references its <see cref="PointId"/>
+/// (SRS-SYN-2/7). Broker PUBACK alone does NOT release it.
+/// </summary>
+[Table("outbound")]
+public sealed class OutboundMessage
+{
+    [PrimaryKey, AutoIncrement] public int Id { get; set; }
+
+    /// <summary>UUIDv7 idempotency key. Unique — a re-enqueue of the same point is a no-op.</summary>
+    [Indexed(Name = "ux_pointid", Order = 1, Unique = true)]
+    public string PointId { get; set; } = "";
+
+    public string Topic { get; set; } = "";
+    public string PayloadJson { get; set; } = "";
+    public string SchemaVersion { get; set; } = "1";
+
+    public long EnqueuedUnixMs { get; set; }
+    public int AttemptCount { get; set; }
+    public long NextAttemptUnixMs { get; set; }
+    public int Status { get; set; } = (int)OutboundStatus.Pending;
+    public string? LastReason { get; set; }
+
+    public static OutboundMessage FromPoint(
+        PointRecord point,
+        string topic,
+        DateTimeOffset enqueuedAt,
+        string? jobId = null,
+        string? ticket = null)
+    {
+        var payload = point.ToAppLogPayloadUtf8(jobId, ticket);
+        return new OutboundMessage
+        {
+            PointId = point.PointId,
+            Topic = topic,
+            PayloadJson = System.Text.Encoding.UTF8.GetString(payload),
+            SchemaVersion = "1",
+            EnqueuedUnixMs = enqueuedAt.ToUnixTimeMilliseconds(),
+            NextAttemptUnixMs = enqueuedAt.ToUnixTimeMilliseconds(),
+            Status = (int)OutboundStatus.Pending,
+        };
+    }
+}
+
+/// <summary>Durable outbound-queue store. Implementations must survive process restart.</summary>
+public interface IOutboundStore
+{
+    Task InitAsync();
+    /// <summary>Enqueue idempotently; returns false if a row with this pointId already exists.</summary>
+    Task<bool> EnqueueAsync(OutboundMessage message);
+    /// <summary>Pending rows whose next-attempt time has arrived, oldest first.</summary>
+    Task<IReadOnlyList<OutboundMessage>> DequeueReadyAsync(DateTimeOffset now, int max = 50);
+    /// <summary>Release a record — accepted by the cloud. Removes it from the queue.</summary>
+    Task ReleaseAckedAsync(string pointId);
+    /// <summary>Record a retryable failure: bump attempt count, schedule the next attempt.</summary>
+    Task RescheduleAsync(string pointId, DateTimeOffset nextAttempt, string reason);
+    /// <summary>Mark a record published-and-awaiting-ack: reschedule a republish (idempotent via
+    /// UUID) after the ack window WITHOUT counting it as a failed attempt.</summary>
+    Task TouchAwaitingAckAsync(string pointId, DateTimeOffset nextAttempt);
+    /// <summary>Terminal rejection (LOG-7): keep the row for surfacing, mark it not-retryable.</summary>
+    Task MarkRejectedAsync(string pointId, string reason);
+    Task<int> PendingCountAsync();
+    Task<IReadOnlyList<OutboundMessage>> GetRejectedAsync();
+}
+
+/// <summary>
+/// SQLite-backed <see cref="IOutboundStore"/>. The DB path is injected (not taken from a
+/// platform API) so it is testable headless with a temp file; reopening the same path is a
+/// process restart. Idempotent enqueue relies on the unique index on pointId.
+/// </summary>
+public sealed class SqliteOutboundStore : IOutboundStore
+{
+    private readonly SQLiteAsyncConnection _db;
+
+    public SqliteOutboundStore(string dbPath)
+    {
+        _db = new SQLiteAsyncConnection(dbPath,
+            SQLiteOpenFlags.ReadWrite | SQLiteOpenFlags.Create | SQLiteOpenFlags.SharedCache);
+    }
+
+    public async Task InitAsync()
+    {
+        await _db.CreateTableAsync<OutboundMessage>();
+        // A process crash can leave rows in-flight after the broker accepted them but before
+        // the application ack was applied. Requeue them; pointId makes the replay idempotent.
+        await _db.ExecuteAsync(
+            "UPDATE outbound SET Status = ?, NextAttemptUnixMs = ? WHERE Status = ?",
+            (int)OutboundStatus.Pending, DateTimeOffset.UtcNow.ToUnixTimeMilliseconds(),
+            (int)OutboundStatus.InFlight);
+    }
+
+    public async Task<bool> EnqueueAsync(OutboundMessage message)
+    {
+        try
+        {
+            await _db.InsertAsync(message);
+            return true;
+        }
+        catch (SQLiteException)
+        {
+            // Unique-index violation on pointId → already queued/known. Idempotent no-op.
+            return false;
+        }
+    }
+
+    public async Task<IReadOnlyList<OutboundMessage>> DequeueReadyAsync(DateTimeOffset now, int max = 50)
+    {
+        long nowMs = now.ToUnixTimeMilliseconds();
+        return await _db.Table<OutboundMessage>()
+            .Where(m => (m.Status == (int)OutboundStatus.Pending || m.Status == (int)OutboundStatus.InFlight)
+                && m.NextAttemptUnixMs <= nowMs)
+            .OrderBy(m => m.EnqueuedUnixMs)
+            .Take(max)
+            .ToListAsync();
+    }
+
+    public Task ReleaseAckedAsync(string pointId) =>
+        _db.ExecuteAsync("DELETE FROM outbound WHERE PointId = ?", pointId);
+
+    public Task RescheduleAsync(string pointId, DateTimeOffset nextAttempt, string reason) =>
+        _db.ExecuteAsync(
+            "UPDATE outbound SET AttemptCount = AttemptCount + 1, NextAttemptUnixMs = ?, LastReason = ? WHERE PointId = ? AND Status = ?",
+            nextAttempt.ToUnixTimeMilliseconds(), reason, pointId, (int)OutboundStatus.Pending);
+
+    public Task TouchAwaitingAckAsync(string pointId, DateTimeOffset nextAttempt) =>
+        _db.ExecuteAsync(
+            "UPDATE outbound SET Status = ?, AttemptCount = AttemptCount + 1, NextAttemptUnixMs = ? WHERE PointId = ? AND Status != ?",
+            (int)OutboundStatus.InFlight, nextAttempt.ToUnixTimeMilliseconds(), pointId,
+            (int)OutboundStatus.RejectedTerminal);
+
+    public Task MarkRejectedAsync(string pointId, string reason) =>
+        _db.ExecuteAsync(
+            "UPDATE outbound SET Status = ?, LastReason = ? WHERE PointId = ?",
+            (int)OutboundStatus.RejectedTerminal, reason, pointId);
+
+    public async Task<int> PendingCountAsync() =>
+        await _db.Table<OutboundMessage>()
+            .Where(m => m.Status == (int)OutboundStatus.Pending || m.Status == (int)OutboundStatus.InFlight)
+            .CountAsync();
+
+    public async Task<IReadOnlyList<OutboundMessage>> GetRejectedAsync() =>
+        await _db.Table<OutboundMessage>().Where(m => m.Status == (int)OutboundStatus.RejectedTerminal).ToListAsync();
+}

src/FieldLogger.Sync/PointRecord.cs

@@ -0,0 +1,250 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace FieldLogger.Sync;
+
+// Wire shape of a Point (SRS §5 / telemetry-schema.md), as published to
+// ul/{orgId}/app/{clientId}/log/points. pointId (UUIDv7) is the idempotency key on every
+// path. This is the app→cloud contract payload; keep field names in sync with
+// telemetry-schema.md (breaking change → backend review).
+
+/// <summary>Point origin (schema Identity group).</summary>
+public enum PointOrigin { APP, LOCATOR, MAGLINK }
+
+/// <summary>How the record reached the cloud (schema Identity group).</summary>
+public enum UploadPath { APP_MQTT, DEVICE_MQTT, REST_BATCH }
+
+/// <summary>What triggered the capture (schema Identity group).</summary>
+public enum CaptureTrigger { APP_UI, LOCATOR_BUTTON }
+
+public sealed record PointRecord
+{
+    /// <summary>Payload schema version — also carried as an MQTT5 user property.</summary>
+    [JsonPropertyName("schemaVersion")] public string SchemaVersion { get; init; } = "1";
+
+    // ---- Identity & provenance ----
+    [JsonPropertyName("pointId")] public required string PointId { get; init; }   // UUIDv7
+    [JsonPropertyName("ticketId")] public string? TicketId { get; init; }
+    [JsonPropertyName("sessionId")] public string? SessionId { get; init; }
+    [JsonPropertyName("pathId")] public string? PathId { get; init; }
+    [JsonPropertyName("category")] public string Category { get; init; } = "LOCATE";
+    [JsonPropertyName("origin")] public PointOrigin Origin { get; init; } = PointOrigin.APP;
+    [JsonPropertyName("originClientId")] public string? OriginClientId { get; init; }
+    [JsonPropertyName("uploadPath")] public UploadPath UploadPath { get; init; } = UploadPath.APP_MQTT;
+    [JsonPropertyName("captureTrigger")] public CaptureTrigger CaptureTrigger { get; init; } = CaptureTrigger.LOCATOR_BUTTON;
+    [JsonPropertyName("createdAt")] public DateTimeOffset CreatedAt { get; init; }
+    [JsonPropertyName("author")] public string? Author { get; init; }
+    [JsonPropertyName("appVersion")] public string? AppVersion { get; init; }
+
+    [JsonPropertyName("position")] public PositionGroup? Position { get; init; }
+    [JsonPropertyName("gnss")] public GnssGroup? Gnss { get; init; }
+    [JsonPropertyName("locate")] public LocateGroup? Locate { get; init; }
+    [JsonPropertyName("attributes")] public AttributesGroup? Attributes { get; init; }
+    [JsonPropertyName("quality")] public QualityGroup? Quality { get; init; }
+
+    private static readonly JsonSerializerOptions JsonOpts = new()
+    {
+        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
+        Converters = { new JsonStringEnumConverter() },
+    };
+
+    public byte[] ToJsonUtf8() => JsonSerializer.SerializeToUtf8Bytes(this, JsonOpts);
+    public static PointRecord FromJsonUtf8(ReadOnlySpan<byte> utf8) =>
+        JsonSerializer.Deserialize<PointRecord>(utf8, JsonOpts)
+        ?? throw new JsonException("null PointRecord");
+
+    /// <summary>
+    /// Serializes a singleton v1 app-log batch matching telemetry-schema.md's frozen MQTT
+    /// profile. Throws a reason-coded exception instead of silently queuing an unusable point.
+    /// </summary>
+    public byte[] ToAppLogPayloadUtf8(string? jobId = null, string? ticket = null)
+    {
+        string? resolvedJobId = string.IsNullOrWhiteSpace(jobId) ? TicketId : jobId;
+        bool hasJob = !string.IsNullOrWhiteSpace(resolvedJobId);
+        bool hasTicket = !string.IsNullOrWhiteSpace(ticket);
+        if (hasJob == hasTicket)
+            throw new PointNotPublishableException("JOB_IDENTITY_INVALID", "Exactly one of jobId or ticket is required.");
+        if (!IsUuidV7(PointId))
+            throw new PointNotPublishableException("POINT_ID_NOT_UUIDV7", "pointId must be a UUIDv7 value.");
+        if (CreatedAt == default)
+            throw new PointNotPublishableException("CREATED_AT_MISSING", "createdAt is required.");
+        if (Position?.Lat is not double lat || Position.Lon is not double lng)
+            throw new PointNotPublishableException("POSITION_MISSING", "Latitude and longitude are required.");
+
+        var wirePoint = new AppLogPointWire
+        {
+            PointId = PointId,
+            CreatedAt = CreatedAt,
+            Origin = "APP",
+            UploadPath = "APP_MQTT",
+            Lat = lat,
+            Lng = lng,
+            Alt = Position.EllipsoidalHeight,
+            Ts = Position.PositionEpoch ?? Locate?.TelemetryEpoch ?? CreatedAt,
+            Fix = NormalizeFix(Gnss?.FixType),
+            HAcc = Gnss?.Hrms,
+            VAcc = Gnss?.Vrms,
+            Sats = Gnss?.SatsUsed,
+            Hdop = Gnss?.Hdop,
+            Depth = Locate?.Depth,
+            FreqHz = Locate?.Frequency is double frequency ? checked((int)Math.Round(frequency)) : null,
+            CurrentMa = Locate?.SignalCurrent,
+            SignalDb = Locate?.SignalStrength,
+            GainDb = Locate?.Gain,
+            Mode = Locate?.LocateMode,
+            PhaseDeg = Locate?.PhaseDegrees,
+            CompassDeg = Locate?.CompassDegrees,
+            DistortionPct = Locate?.DistortionPercent,
+            Utility = NormalizeUtility(Attributes?.UtilityType),
+            QualityFlag = Quality?.QualityFlag ?? "IN_SPEC",
+        };
+        var envelope = new AppLogPointsEnvelope
+        {
+            SchemaVersion = "1",
+            JobId = hasJob ? resolvedJobId : null,
+            Ticket = hasTicket ? ticket : null,
+            Points = [wirePoint],
+        };
+        return JsonSerializer.SerializeToUtf8Bytes(envelope, AppLogJsonOptions);
+    }
+
+    private static bool IsUuidV7(string value)
+    {
+        string text = value.ToLowerInvariant();
+        return Guid.TryParseExact(text, "D", out _)
+            && text.Length == 36
+            && text[14] == '7'
+            && text[19] is '8' or '9' or 'a' or 'b';
+    }
+
+    private static string? NormalizeFix(string? value) => value?.ToUpperInvariant() switch
+    {
+        null or "" => null,
+        "AUTONOMOUS" => "AUTONOMOUS",
+        "DGPS" => "DGPS",
+        "FLOAT" or "RTK_FLOAT" or "FLOAT_RTK" => "FLOAT",
+        "FIXED" or "RTK_FIXED" or "FIXED_RTK" => "FIXED",
+        "NO_FIX" or "NONE" => "NO_FIX",
+        _ => throw new PointNotPublishableException("FIX_TYPE_INVALID", $"Unsupported fix type '{value}'."),
+    };
+
+    private static string? NormalizeUtility(string? value) => value?.ToUpperInvariant() switch
+    {
+        null or "" => null,
+        "ELECTRIC" or "GAS" or "WATER" or "SEWER" or "TELECOM" or "CATV" or "FIBER" or "STEAM" or "UNKNOWN"
+            => value.ToUpperInvariant(),
+        _ => throw new PointNotPublishableException("UTILITY_TYPE_INVALID", $"Unsupported utility type '{value}'."),
+    };
+
+    private static readonly JsonSerializerOptions AppLogJsonOptions = new()
+    {
+        PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
+        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
+    };
+}
+
+public sealed class PointNotPublishableException : InvalidOperationException
+{
+    public string ReasonCode { get; }
+
+    public PointNotPublishableException(string reasonCode, string message) : base(message) =>
+        ReasonCode = reasonCode;
+}
+
+internal sealed record AppLogPointsEnvelope
+{
+    public string SchemaVersion { get; init; } = "1";
+    public string? JobId { get; init; }
+    public string? Ticket { get; init; }
+    public List<AppLogPointWire> Points { get; init; } = [];
+}
+
+internal sealed record AppLogPointWire
+{
+    public required string PointId { get; init; }
+    public required DateTimeOffset CreatedAt { get; init; }
+    public string Origin { get; init; } = "APP";
+    public string UploadPath { get; init; } = "APP_MQTT";
+    public required double Lat { get; init; }
+    public required double Lng { get; init; }
+    public double? Alt { get; init; }
+    public required DateTimeOffset Ts { get; init; }
+    public string? Fix { get; init; }
+    public double? HAcc { get; init; }
+    public double? VAcc { get; init; }
+    public int? Sats { get; init; }
+    public double? Hdop { get; init; }
+    public double? Depth { get; init; }
+    public int? FreqHz { get; init; }
+    public double? CurrentMa { get; init; }
+    public double? SignalDb { get; init; }
+    public double? GainDb { get; init; }
+    public string? Mode { get; init; }
+    public double? PhaseDeg { get; init; }
+    public double? CompassDeg { get; init; }
+    public double? DistortionPct { get; init; }
+    public string? Utility { get; init; }
+    public string QualityFlag { get; init; } = "IN_SPEC";
+}
+
+public sealed record PositionGroup
+{
+    [JsonPropertyName("lat")] public double? Lat { get; init; }
+    [JsonPropertyName("lon")] public double? Lon { get; init; }
+    [JsonPropertyName("crsEpsg")] public int? CrsEpsg { get; init; }
+    [JsonPropertyName("ellipsoidalHeight")] public double? EllipsoidalHeight { get; init; }
+    [JsonPropertyName("orthometricHeight")] public double? OrthometricHeight { get; init; }
+    [JsonPropertyName("geoidModel")] public string? GeoidModel { get; init; }
+    [JsonPropertyName("antennaHeight")] public double? AntennaHeight { get; init; }
+    [JsonPropertyName("positionEpoch")] public DateTimeOffset? PositionEpoch { get; init; }
+}
+
+public sealed record GnssGroup
+{
+    [JsonPropertyName("fixType")] public string? FixType { get; init; }
+    [JsonPropertyName("satsUsed")] public int? SatsUsed { get; init; }
+    [JsonPropertyName("hdop")] public double? Hdop { get; init; }
+    [JsonPropertyName("hrms")] public double? Hrms { get; init; }
+    [JsonPropertyName("vrms")] public double? Vrms { get; init; }
+    [JsonPropertyName("correctionAge")] public double? CorrectionAge { get; init; }
+    [JsonPropertyName("receiverModel")] public string? ReceiverModel { get; init; }
+    [JsonPropertyName("receiverSerial")] public string? ReceiverSerial { get; init; }
+    [JsonPropertyName("source")] public string? Source { get; init; }   // MAGLINK | PHONE
+    [JsonPropertyName("tiltAngle")] public double? TiltAngle { get; init; }
+    [JsonPropertyName("clockSource")] public string? ClockSource { get; init; }
+}
+
+public sealed record LocateGroup
+{
+    [JsonPropertyName("depth")] public double? Depth { get; init; }
+    [JsonPropertyName("depthUnits")] public string? DepthUnits { get; init; }
+    [JsonPropertyName("signalCurrent")] public double? SignalCurrent { get; init; }
+    [JsonPropertyName("signalStrength")] public double? SignalStrength { get; init; }
+    [JsonPropertyName("frequency")] public double? Frequency { get; init; }
+    [JsonPropertyName("locateMode")] public string? LocateMode { get; init; }
+    [JsonPropertyName("gain")] public double? Gain { get; init; }
+    [JsonPropertyName("signalDirection")] public double? SignalDirection { get; init; }
+    [JsonPropertyName("phaseDegrees")] public double? PhaseDegrees { get; init; }
+    [JsonPropertyName("compassDegrees")] public double? CompassDegrees { get; init; }
+    [JsonPropertyName("distortionPercent")] public double? DistortionPercent { get; init; }
+    [JsonPropertyName("warningFlags")] public int? WarningFlags { get; init; }
+    [JsonPropertyName("locatorModel")] public string? LocatorModel { get; init; }
+    [JsonPropertyName("locatorSerial")] public string? LocatorSerial { get; init; }
+    [JsonPropertyName("telemetryEpoch")] public DateTimeOffset? TelemetryEpoch { get; init; }
+}
+
+public sealed record AttributesGroup
+{
+    [JsonPropertyName("utilityType")] public string? UtilityType { get; init; }
+    [JsonPropertyName("owner")] public string? Owner { get; init; }
+    [JsonPropertyName("markerColor")] public string? MarkerColor { get; init; }
+    [JsonPropertyName("surfaceType")] public string? SurfaceType { get; init; }
+    [JsonPropertyName("notes")] public string? Notes { get; init; }
+}
+
+public sealed record QualityGroup
+{
+    [JsonPropertyName("qualityFlag")] public string? QualityFlag { get; init; }
+    [JsonPropertyName("gatePolicyId")] public string? GatePolicyId { get; init; }
+    [JsonPropertyName("waiverId")] public string? WaiverId { get; init; }
+}

tests/FieldLogger.Sync.Tests/FakeMqttTransport.cs

@@ -0,0 +1,61 @@
+using FieldLogger.Sync;
+
+namespace FieldLogger.Sync.Tests;
+
+/// <summary>
+/// In-memory stand-in for the broker. Records published payloads, lets a test toggle
+/// connectivity, force publish failures, and inject application-level acks — so the engine's
+/// durability/ack/backoff behaviour is exercised without MQTTnet or a live broker.
+/// </summary>
+public sealed class FakeMqttTransport : IMqttTransport
+{
+    public bool IsConnected { get; set; } = true;
+    public bool FailNextPublish { get; set; }
+    public List<(string Topic, byte[] Payload)> Published { get; } = new();
+
+    public event Func<AckBatch, Task>? AckReceived;
+
+    public Task ConnectAsync(string ackTopic, CancellationToken ct = default)
+    {
+        IsConnected = true;
+        return Task.CompletedTask;
+    }
+
+    public Task DisconnectAsync()
+    {
+        IsConnected = false;
+        return Task.CompletedTask;
+    }
+
+    public Task PublishAsync(string topic, byte[] payload, string schemaVersion, CancellationToken ct = default)
+    {
+        if (!IsConnected) throw new InvalidOperationException("not connected");
+        if (FailNextPublish)
+        {
+            FailNextPublish = false;
+            throw new InvalidOperationException("simulated broker publish failure");
+        }
+        Published.Add((topic, payload));
+        return Task.CompletedTask;
+    }
+
+    /// <summary>Simulate the cloud emitting an application-level ack for these pointIds.</summary>
+    public async Task InjectAckAsync(params AckItem[] items)
+    {
+        if (AckReceived is { } handlers)
+        {
+            var batch = new AckBatch { Results = items.ToList() };
+            foreach (Func<AckBatch, Task> handler in handlers.GetInvocationList())
+                await handler(batch);
+        }
+    }
+
+    public Task InjectAcceptAsync(string pointId) =>
+        InjectAckAsync(new AckItem { PointId = pointId, Outcome = AckOutcome.ACCEPTED });
+
+    public Task InjectDuplicateAsync(string pointId) =>
+        InjectAckAsync(new AckItem { PointId = pointId, Outcome = AckOutcome.DUPLICATE });
+
+    public Task InjectRejectAsync(string pointId, string reasonCode) =>
+        InjectAckAsync(new AckItem { PointId = pointId, Outcome = AckOutcome.REJECTED, ReasonCode = reasonCode });
+}

tests/FieldLogger.Sync.Tests/FieldLogger.Sync.Tests.csproj

@@ -0,0 +1,35 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <!--
+    Headless tests for the cloud-sync core (S2-b/S2-c): durable-queue restart survival,
+    ack-release, exactly-once, backoff, broker-loss-safety, and LOG-7 no-silent-discard.
+    Plain net9.0 + xUnit so it runs in the QA gate (auto-discovered *.Tests.csproj) without
+    MAUI workloads or a live broker (uses a fake IMqttTransport).
+    Run: dotnet test tests/FieldLogger.Sync.Tests/FieldLogger.Sync.Tests.csproj
+  -->
+
+  <PropertyGroup>
+    <TargetFramework>net9.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+    <IsTestProject>true</IsTestProject>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.11.1" />
+    <PackageReference Include="xunit" Version="2.9.2" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\FieldLogger.Sync\FieldLogger.Sync.csproj" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <None Include="..\..\..\meta\contracts\fixtures\app-log-*.json"
+          Link="ContractFixtures\%(Filename)%(Extension)"
+          CopyToOutputDirectory="PreserveNewest" />
+  </ItemGroup>
+
+</Project>

tests/FieldLogger.Sync.Tests/SyncEngineTests.cs

@@ -0,0 +1,173 @@
+using System.Text.Json;
+using System.Text.Json.Nodes;
+using FieldLogger.Sync;
+using Xunit;
+
+namespace FieldLogger.Sync.Tests;
+
+public sealed class SyncEngineTests
+{
+    private static readonly DateTimeOffset Now = DateTimeOffset.Parse("2026-08-21T10:00:00Z");
+
+    static SyncEngineTests() => SQLitePCL.Batteries_V2.Init();
+
+    [Fact]
+    public void Point_serializes_to_the_frozen_singleton_batch()
+    {
+        var point = MakePoint("018f1a00-0000-7000-8000-000000000001");
+        using var json = JsonDocument.Parse(point.ToAppLogPayloadUtf8(jobId: "job_1"));
+
+        var root = json.RootElement;
+        Assert.Equal("1", root.GetProperty("schemaVersion").GetString());
+        Assert.Equal("job_1", root.GetProperty("jobId").GetString());
+        var wire = Assert.Single(root.GetProperty("points").EnumerateArray());
+        Assert.Equal(point.PointId, wire.GetProperty("pointId").GetString());
+        Assert.Equal("APP", wire.GetProperty("origin").GetString());
+        Assert.Equal("APP_MQTT", wire.GetProperty("uploadPath").GetString());
+        Assert.Equal("FIXED", wire.GetProperty("fix").GetString());
+        Assert.Equal(-80.2, wire.GetProperty("lng").GetDouble());
+    }
+
+    [Fact]
+    public void App_serializer_matches_the_shared_contract_fixture()
+    {
+        JsonNode? actual = JsonNode.Parse(MakePoint("018f1a00-0000-7000-8000-000000000001")
+            .ToAppLogPayloadUtf8(jobId: "job_1"));
+        string path = Path.Combine(AppContext.BaseDirectory, "ContractFixtures", "app-log-points-v1.json");
+        JsonNode? expected = JsonNode.Parse(File.ReadAllText(path));
+
+        Assert.True(JsonNode.DeepEquals(expected, actual));
+    }
+
+    [Fact]
+    public void Invalid_capture_is_refused_with_a_reason_code()
+    {
+        var point = MakePoint("018f1a00-0000-7000-8000-000000000002") with { Position = null };
+        var error = Assert.Throws<PointNotPublishableException>(() => point.ToAppLogPayloadUtf8(jobId: "job_1"));
+        Assert.Equal("POSITION_MISSING", error.ReasonCode);
+    }
+
+    [Fact]
+    public async Task Queue_survives_restart_and_releases_only_after_acceptance()
+    {
+        string db = NewDbPath();
+        try
+        {
+            var firstStore = new SqliteOutboundStore(db);
+            await firstStore.InitAsync();
+            var message = OutboundMessage.FromPoint(
+                MakePoint("018f1a00-0000-7000-8000-000000000003"),
+                "ul/org_alpha/app/client_1/log/points", Now, jobId: "job_1");
+            Assert.True(await firstStore.EnqueueAsync(message));
+
+            var firstTransport = new FakeMqttTransport();
+            var firstEngine = Engine(firstTransport, firstStore);
+            Assert.Equal(1, await firstEngine.DrainOnceAsync());
+            Assert.Equal(1, await firstStore.PendingCountAsync()); // broker PUBACK is not enough
+
+            var restartedStore = new SqliteOutboundStore(db);
+            await restartedStore.InitAsync(); // resets interrupted in-flight work to pending
+            var restartedTransport = new FakeMqttTransport();
+            var restartedEngine = Engine(restartedTransport, restartedStore);
+            Assert.Equal(1, await restartedEngine.DrainOnceAsync());
+            await restartedTransport.InjectAcceptAsync(message.PointId);
+
+            Assert.Equal(0, await restartedStore.PendingCountAsync());
+        }
+        finally
+        {
+            DeleteDb(db);
+        }
+    }
+
+    [Fact]
+    public async Task Duplicate_ack_is_safe_to_release_and_publish_failure_is_not()
+    {
+        string db = NewDbPath();
+        try
+        {
+            var store = new SqliteOutboundStore(db);
+            await store.InitAsync();
+            var message = OutboundMessage.FromPoint(
+                MakePoint("018f1a00-0000-7000-8000-000000000004"),
+                "ul/org_alpha/app/client_1/log/points", Now, jobId: "job_1");
+            Assert.True(await store.EnqueueAsync(message));
+
+            var transport = new FakeMqttTransport { FailNextPublish = true };
+            var engine = Engine(transport, store);
+            Assert.Equal(0, await engine.DrainOnceAsync());
+            Assert.Equal(1, await store.PendingCountAsync());
+
+            await transport.InjectDuplicateAsync(message.PointId);
+            Assert.Equal(0, await store.PendingCountAsync());
+        }
+        finally
+        {
+            DeleteDb(db);
+        }
+    }
+
+    [Fact]
+    public async Task Terminal_rejection_is_retained_and_surfaced()
+    {
+        string db = NewDbPath();
+        try
+        {
+            var store = new SqliteOutboundStore(db);
+            await store.InitAsync();
+            var message = OutboundMessage.FromPoint(
+                MakePoint("018f1a00-0000-7000-8000-000000000005"),
+                "ul/org_alpha/app/client_1/log/points", Now, jobId: "job_1");
+            await store.EnqueueAsync(message);
+            var transport = new FakeMqttTransport();
+            var engine = Engine(transport, store);
+            OutboundMessage? surfaced = null;
+            engine.PointRejected += value =>
+            {
+                surfaced = value;
+                return Task.CompletedTask;
+            };
+
+            await transport.InjectRejectAsync(message.PointId, "VALIDATION_ERROR");
+
+            Assert.Equal(0, await store.PendingCountAsync());
+            Assert.Equal(message.PointId, surfaced?.PointId);
+            Assert.Equal("VALIDATION_ERROR", Assert.Single(await store.GetRejectedAsync()).LastReason);
+        }
+        finally
+        {
+            DeleteDb(db);
+        }
+    }
+
+    [Fact]
+    public void Ack_parser_uses_results_and_outcome()
+    {
+        var ack = AckBatch.FromJsonUtf8(
+            """{"schemaVersion":"1","results":[{"pointId":"018f1a00-0000-7000-8000-000000000006","outcome":"DUPLICATE"}]}"""u8);
+        Assert.Equal(AckOutcome.DUPLICATE, Assert.Single(ack.Results).Outcome);
+    }
+
+    private static MqttSyncEngine Engine(FakeMqttTransport transport, IOutboundStore store) =>
+        new(transport, store, new SyncOptions { OrgId = "org_alpha", ClientId = "client_1" },
+            () => Now, new BackoffPolicy(TimeSpan.FromSeconds(1), TimeSpan.FromMinutes(1), new Random(1)));
+
+    private static PointRecord MakePoint(string id) => new()
+    {
+        PointId = id,
+        CreatedAt = Now,
+        Position = new PositionGroup { Lat = 40.1, Lon = -80.2, PositionEpoch = Now.AddSeconds(-1) },
+        Gnss = new GnssGroup { FixType = "RTK_FIXED", SatsUsed = 18, Hrms = 0.02, Vrms = 0.04 },
+        Attributes = new AttributesGroup { UtilityType = "WATER" },
+    };
+
+    private static string NewDbPath() => Path.Combine(Path.GetTempPath(), $"fieldlogger-sync-{Guid.NewGuid():N}.db3");
+
+    private static void DeleteDb(string db)
+    {
+        foreach (string path in new[] { db, $"{db}-shm", $"{db}-wal" })
+        {
+            if (File.Exists(path)) File.Delete(path);
+        }
+    }
+}