test: own Avalonia application session

tests/AcDream.Launcher.Tests/TestAppBuilder.cs

@@ -2,6 +2,7 @@ using Avalonia;
 using Avalonia.Headless;
 
 [assembly: AvaloniaTestApplication(typeof(AcDream.Launcher.Tests.TestAppBuilder))]
+[assembly: AvaloniaTestIsolation(AvaloniaTestIsolationLevel.PerAssembly)]
 
 namespace AcDream.Launcher.Tests;
 
@@ -12,6 +13,11 @@ namespace AcDream.Launcher.Tests;
 /// live for every test — <see cref="MainWindow"/>'s controls (TreeView,
 /// Button, TextBox, ...) need a real control theme to template and focus
 /// correctly, not just an unstyled visual tree.
+/// The assembly contains one Avalonia test that owns the complete window
+/// matrix. Keep one application/dispatcher for that process: Avalonia 12's
+/// per-test teardown can recreate the compositor after its render loop has
+/// become owned by a different worker, producing a thread-affinity cleanup
+/// failure after an otherwise passing test.
 /// </summary>
 public static class TestAppBuilder
 {