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

ytLive.Tests/RoundClipInteractionTests.cs

@@ -5,6 +5,7 @@ using System.Windows.Controls;
 using System.Windows.Media;
 using System.Windows.Shapes;
 using System.Windows.Threading;
+using Microsoft.Data.Sqlite;
 using Xunit;
 using ytLive.Models;
 using ytLive.ViewModels;
@@ -47,6 +48,11 @@ public sealed class RoundClipInteractionTests
         var app = new App();
         app.InitializeComponent();
 
+        // Never let the real MainWindow read/write the user's actual layout DB —
+        // Shutdown() saves the layout, which would persist these test sources
+        // over the real ones. Point it at a throwaway temp DB instead.
+        var tempDb = System.IO.Path.Combine(System.IO.Path.GetTempPath(), $"ytLlive-test-{Guid.NewGuid():N}.db");
+        MainViewModel.LayoutPathOverride = tempDb;
         var window = new MainWindow();
         var vm = (MainViewModel)window.DataContext;
         try
@@ -103,6 +109,9 @@ public sealed class RoundClipInteractionTests
         finally
         {
             window.Close();
+            MainViewModel.LayoutPathOverride = null;
+            SqliteConnection.ClearAllPools();
+            try { System.IO.File.Delete(tempDb); } catch { /* best-effort cleanup */ }
         }
     }