Write settings, presets and favourites whole or not at all

A failed write leaves the old file in place. Settings that could not be read
are not saved over. UI exceptions reach the log. Started from a shortcut set
up elsewhere, the starter moves to its own folder when only that one holds a
game.

StarterNG.Tests/Fakes/TestLog.cs

@@ -0,0 +1,12 @@
+using StarterNG.Application.Abstractions;
+using StarterNG.Infrastructure.Adapters;
+
+namespace StarterNG.Tests.Fakes;
+
+/// <summary>The diagnostics log for a test that only wants one to write to or read back.</summary>
+public static class TestLog
+{
+    /// <param name="files">Where the log file goes; a disk of its own when left out.</param>
+    public static FileDiagnosticsLog Over(IFileSystem? files = null) =>
+        new(files ?? new InMemoryFileSystem(), new FixedClock(new DateTime(2026, 10, 13)), new GamePaths("game"));
+}