Webcam resource validation + first-frame proof + CameraConflictProbe: MediaCaptureFrameSource validates post-init (VideoDeviceId match, stream properties, reader StartAsync status), subscribes Failed/CameraStreamStateChanged → SourceFailed; CameraManager.AcquireAsync requires first-frame proof (4s timeout) — silent empty box impossible; MainViewModel surfaces WebcamError chip + MessageBox with suspect-app names; fallback ladder VideoPreview → VideoRecord; 19041 SDK projection gaps documented; AGENTS.md + schema.md + HANDOFF.md memory-map rules added; 81 tests passing, 0 warnings

Services/ICameraFrameSource.cs

@@ -8,7 +8,16 @@ namespace ytLive.Services;
 public interface ICameraFrameSource
 {
     string DeviceId { get; }
+
+    /// <summary>Normalized BGRA frames from a worker thread; callers marshal to the UI thread.</summary>
     event Action<VideoFrame>? FrameAvailable;
+
+    /// <summary>
+    /// Raised when the capture session fails asynchronously after a successful start
+    /// (device lost, stream state Failed, media capture failure). Carries the reason.
+    /// </summary>
+    event Action<string>? SourceFailed;
+
     Task StartAsync();
     Task StopAsync();
 }