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/SmokeTests.cs

@@ -0,0 +1,24 @@
+using Xunit;
+
+namespace FieldLogger.Tests;
+
+// Smoke test for the QA gate (S1-f, trace: SRS §6 gate / NFR-8).
+// Proves the .NET test toolchain (dotnet test + xUnit) is wired and green in the app repo.
+// Substantive coverage (e.g. the S1-b IF-LOC codec round-trip) is added once the app's
+// pure logic is factored into a workload-free library this project can reference.
+public class SmokeTests
+{
+    [Fact]
+    public void Harness_Runs()
+    {
+        Assert.True(true);
+    }
+
+    [Theory]
+    [InlineData(1, 1, 2)]
+    [InlineData(2, 3, 5)]
+    public void Arithmetic_Sanity(int a, int b, int expected)
+    {
+        Assert.Equal(expected, a + b);
+    }
+}