Introduce layers, ports and a composition root

Ports in Application/Abstractions, adapters in Infrastructure, hand-wired
AppServices (no reflection, as PublishAot requires). Adds a test project.

StarterNG.Tests/FileDiagnosticsLogTests.cs

@@ -0,0 +1,45 @@
+using StarterNG.Infrastructure.Adapters;
+using StarterNG.Tests.Fakes;
+
+namespace StarterNG.Tests;
+
+public class FileDiagnosticsLogTests
+{
+    private static FileDiagnosticsLog Build(out InMemoryFileSystem files)
+    {
+        files = new InMemoryFileSystem();
+        return new FileDiagnosticsLog(files, new FixedClock(new DateTime(2026, 3, 7, 14, 5, 0)),
+                                      new GamePaths("game"));
+    }
+
+    [Fact]
+    public void Log_timestamps_each_line_from_the_clock()
+    {
+        var log = Build(out _);
+
+        log.Log("scenery loaded");
+
+        Assert.Equal("2026-03-07 14:05:00  scenery loaded", log.Text);
+    }
+
+    [Fact]
+    public void Exceptions_are_logged_with_their_context_and_type()
+    {
+        var log = Build(out _);
+
+        log.Log("scenery/foo.scn", new InvalidOperationException("bad node"));
+
+        Assert.Contains("scenery/foo.scn: InvalidOperationException: bad node", log.Text);
+    }
+
+    [Fact]
+    public void Clear_drops_everything_logged_so_far()
+    {
+        var log = Build(out _);
+        log.Log("first");
+
+        log.Clear();
+
+        Assert.Equal(string.Empty, log.Text);
+    }
+}