Screen backdrop (schema v5/v6) + five-scene catalog + webcam polish: live desktop/game capture as a permanent non-deletable bottom layer (Source.IsBackdrop), auto full-screen game detection (Win32FullScreenDetector) else primary display, GraphicsCapturePicker re-designation, refcounted shared ScreenCaptureManager, Live-only backdrop by policy (HasBackdrop + v5-v6 backfill + EnforceBackdropPolicy, checkbox gone), SceneCatalog (Starting/Live/BRB/Chat/Ending) with + button re-adding missing scenes, webcam mid-session transparent-container fix (GetPreviewBitmap propagation), Chat half-screen-area cap, 'Add Webcam' always opens the picker (SwapWebcamIdentityAsync, no silent resurrect), WindowsRuntimeMarshal frame-read + 5s-throttled errors, docs updated, tests (65 passing)

Services/IScreenCaptureSource.cs

@@ -0,0 +1,15 @@
+namespace ytLive.Services;
+
+/// <summary>
+/// A running screen-capture source for one target key ("monitor:&lt;n&gt;" or
+/// "window:&lt;hwnd&gt;"). Raises normalized BGRA frames from a worker thread;
+/// callers must marshal to the UI thread. Seam so ScreenCaptureManager is
+/// testable without WinRT.
+/// </summary>
+public interface IScreenCaptureSource
+{
+    string Key { get; }
+    event Action<VideoFrame>? FrameAvailable;
+    Task StartAsync();
+    Task StopAsync();
+}