test: stand up xUnit harness for QA gate (S1-f)

Standalone net9.0 xUnit project at tests/FieldLogger.Tests (no MAUI workload)
so `dotnet test` runs fast in the QA gate. Smoke suite only for now; real app
logic tests land once codec/sim are factored into a workload-free library.

Trace: SRS §6 gate, NFR-8.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/FieldLogger.Tests/FieldLogger.Tests.csproj

@@ -0,0 +1,28 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <!--
+    QA smoke/regression harness for the Field Logger app (S1-f, trace: SRS §6 gate / NFR-8).
+
+    Targets plain net9.0 (NOT the MAUI platform TFMs) so `dotnet test` runs on any dev/CI
+    machine without the MAUI workload, keeping the QA gate fast. It deliberately does NOT
+    reference FieldLogger.csproj yet, because that project pulls in MAUI/platform deps.
+
+    When app logic that needs testing (e.g. the S1-b IF-LOC codec / locator simulator) is
+    factored into a plain .NET class library, add a ProjectReference to it here and the
+    round-trip tests move in. Coordinated with app-owner.
+  -->
+  <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>
+
+</Project>