TASK 3 milestone 1: webcam capture (MediaCapture CPU-first, CameraManager refcount, picker, clip/mirror, schema v2, tests)

Services/ICameraFrameSource.cs

@@ -0,0 +1,14 @@
+namespace ytLive.Services;
+
+/// <summary>
+/// A running capture source for one device. Raises normalized BGRA frames from
+/// a worker thread; callers must marshal to the UI thread. Seam so CameraManager
+/// is testable without WinRT.
+/// </summary>
+public interface ICameraFrameSource
+{
+    string DeviceId { get; }
+    event Action<VideoFrame>? FrameAvailable;
+    Task StartAsync();
+    Task StopAsync();
+}