Preview: promote selected layer for hit testing

Temporarily raise the selected preview item and prioritize it in the custom hit test without changing scene order. Refresh integration-test expectations for Brand logo layers and explicit thumbnail assets.

WPF references: https://learn.microsoft.com/en-us/dotnet/api/system.windows.controls.panel.zindex?view=windowsdesktop-10.0 https://learn.microsoft.com/en-us/dotnet/desktop/wpf/graphics-multimedia/hit-testing-in-the-visual-layer

ytLive.Tests/SourceNamingTests.cs

@@ -44,8 +44,8 @@ public sealed class SourceNamingTests
             vm.AddSourceCommand.Execute(SourceType.TextOverlay);
             vm.AddSourceCommand.Execute(SourceType.TextOverlay);
 
-            // The locked Background layer is always present but never numbered.
-            var names = scene.Elements.OfType<Source>().Where(s => !s.IsBackground)
+            // Persistent scene layers are not part of the user-created Text sequence.
+            var names = scene.Elements.OfType<Source>().Where(s => s.Type == SourceType.TextOverlay)
                 .Select(s => s.Name).ToList();
             Assert.Equal(new[] { "Text", "Text2", "Text3" }, names);
 
@@ -53,7 +53,7 @@ public sealed class SourceNamingTests
             scene.Elements.Remove(middle);
             vm.AddSourceCommand.Execute(SourceType.TextOverlay);
 
-            var survivors = scene.Elements.OfType<Source>().Where(s => !s.IsBackground)
+            var survivors = scene.Elements.OfType<Source>().Where(s => s.Type == SourceType.TextOverlay)
                 .Select(s => s.Name).ToList();
             Assert.Equal(new[] { "Text", "Text3", "Text2" }, survivors);
         }