Initial commit: FieldLogger MAUI app with Maglink BLE support

Added Maglink RTK GNSS receiver integration with correct BLE UUIDs and device name filtering (ML-* prefix).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

FieldLogger/Models/GnssFix.cs

@@ -0,0 +1,155 @@
+using System.Globalization;
+
+namespace FieldLogger.Models;
+
+public enum GnssFixStatus { NoFix = 0, Single = 1, Dgps = 2, RtkFixed = 4, RtkFloat = 5 }
+
+/// <summary>
+/// A parsed $GNPOS sentence from the Maglink (H11) RTK receiver.
+/// </summary>
+public sealed class GnssFix
+{
+    public double Latitude { get; init; }
+    public double Longitude { get; init; }
+    public double Altitude { get; init; }
+    public double AltitudeCorrected { get; init; }
+    public GnssFixStatus Status { get; init; }
+    public double Hdop { get; init; }
+    public double Hrms { get; init; }
+    public double Vrms { get; init; }
+    public int SatellitesUsed { get; init; }
+    public int SatellitesVisible { get; init; }
+    public double SpeedKmh { get; init; }
+    public double Heading { get; init; }
+    public double BatteryVoltage { get; init; }
+    public int BatteryPercent { get; init; }
+    public bool NtripConnected { get; init; }
+    public int RtcmSize { get; init; }
+    public double CorrectionAgeSeconds { get; init; }
+    public long UnixTimestamp { get; init; }
+    public double TiltAngle { get; init; }
+    public string RawSentence { get; init; } = "";
+
+    /// <summary>Local UTC time this fix was received by the app.</summary>
+    public DateTime ReceivedUtc { get; init; } = DateTime.UtcNow;
+
+    public bool HasPosition => Status != GnssFixStatus.NoFix;
+
+    public string StatusLabel => Status switch
+    {
+        GnssFixStatus.NoFix => "No Fix",
+        GnssFixStatus.Single => "Single",
+        GnssFixStatus.Dgps => "DGPS",
+        GnssFixStatus.RtkFixed => "RTK Fixed",
+        GnssFixStatus.RtkFloat => "RTK Float",
+        _ => Status.ToString(),
+    };
+
+    /// <summary>Parses a "$GNPOS,..." sentence (checksum must already be validated).</summary>
+    public static GnssFix? TryParse(string sentence)
+    {
+        var body = NmeaSentence.Body(sentence);
+        var f = body.Split(',');
+        if (f.Length < 20 || f[0] != "GNPOS")
+            return null;
+
+        try
+        {
+            return new GnssFix
+            {
+                Latitude = D(f[1]),
+                Longitude = D(f[2]),
+                Altitude = D(f[3]),
+                AltitudeCorrected = D(f[4]),
+                Status = (GnssFixStatus)I(f[5]),
+                Hdop = D(f[6]),
+                Hrms = D(f[7]),
+                Vrms = D(f[8]),
+                SatellitesUsed = I(f[9]),
+                SatellitesVisible = I(f[10]),
+                SpeedKmh = D(f[11]),
+                Heading = D(f[12]),
+                BatteryVoltage = D(f[13]),
+                BatteryPercent = I(f[14]),
+                NtripConnected = I(f[15]) != 0,
+                RtcmSize = I(f[16]),
+                CorrectionAgeSeconds = D(f[17]),
+                UnixTimestamp = long.Parse(f[18], CultureInfo.InvariantCulture),
+                TiltAngle = D(f[19]),
+                RawSentence = sentence.Trim(),
+            };
+        }
+        catch (FormatException)
+        {
+            return null;
+        }
+    }
+
+    private static double D(string s) => double.Parse(s, NumberStyles.Float, CultureInfo.InvariantCulture);
+    private static int I(string s) => int.Parse(s, CultureInfo.InvariantCulture);
+}
+
+/// <summary>
+/// A parsed $GNDEV device-information sentence from the Maglink receiver.
+/// </summary>
+public sealed class GnssDeviceInfo
+{
+    public string SerialNumber { get; init; } = "";
+    public string PcbVersion { get; init; } = "";
+    public string FirmwareVersion { get; init; } = "";
+    public string Imei { get; init; } = "";
+    public string Imsi { get; init; } = "";
+    public string Iccid { get; init; } = "";
+
+    public static GnssDeviceInfo? TryParse(string sentence)
+    {
+        var f = NmeaSentence.Body(sentence).Split(',');
+        if (f.Length < 7 || f[0] != "GNDEV")
+            return null;
+
+        return new GnssDeviceInfo
+        {
+            SerialNumber = f[1],
+            PcbVersion = f[2],
+            FirmwareVersion = f[3],
+            Imei = f[4],
+            Imsi = f[5],
+            Iccid = f[6],
+        };
+    }
+}
+
+/// <summary>NMEA framing helpers ($...*hh checksum).</summary>
+public static class NmeaSentence
+{
+    /// <summary>Returns the sentence body between '$' and '*' (or end of string).</summary>
+    public static string Body(string sentence)
+    {
+        var s = sentence.Trim().TrimStart('$');
+        var star = s.IndexOf('*');
+        return star >= 0 ? s[..star] : s;
+    }
+
+    public static byte Checksum(string sentence)
+    {
+        byte checksum = 0;
+        foreach (var c in Body(sentence))
+            checksum ^= (byte)c;
+        return checksum;
+    }
+
+    /// <summary>Validates the trailing *hh checksum. Sentences without a checksum fail validation.</summary>
+    public static bool VerifyChecksum(string sentence)
+    {
+        var star = sentence.LastIndexOf('*');
+        if (star < 0 || star + 3 > sentence.TrimEnd().Length)
+            return false;
+
+        var hex = sentence.Substring(star + 1, 2);
+        if (!byte.TryParse(hex, System.Globalization.NumberStyles.HexNumber,
+                System.Globalization.CultureInfo.InvariantCulture, out var expected))
+            return false;
+
+        return Checksum(sentence) == expected;
+    }
+}