Hermetic real-MainWindow tests: LayoutPathOverride temp-DB seam (InternalsVisibleTo); fix tests persisting fake webcam sources over the real layout DB; LayoutStore delete-roundtrip guard

ViewModels/MainViewModel.cs

@@ -481,7 +481,7 @@ public class MainViewModel : ViewModelBase
         SaveLayoutAsCommand = new RelayCommand(_ => SaveLayoutAs());
         OpenLayoutCommand = new RelayCommand(_ => OpenLayoutFile());
 
-        _layoutStore = new LayoutStore(DefaultLayoutPath);
+        _layoutStore = new LayoutStore(LayoutPathOverride ?? DefaultLayoutPath);
         _activeLayoutPath = _layoutStore.ActivePath;
 
         _cameraEnumerator = new MediaCaptureCameraEnumerator();
@@ -531,6 +531,11 @@ public class MainViewModel : ViewModelBase
         "ytLlive",
         "ytLlive.db");
 
+    // Test seam: a real MainWindow test must never read or write the user's
+    // real layout DB (it would persist test sources over real ones). Tests set
+    // this to a temp path before constructing MainWindow and reset it after.
+    internal static string? LayoutPathOverride { get; set; }
+
     private void LoadLayout()
     {
         _isLoading = true;