feat: add webcam frame diagnostics to debug gray block issue

Co-authored-by: aider (openrouter/anthropic/claude-sonnet-4) <aider@aider.chat>

Services/Compositor/SceneCompositor.cs

@@ -257,6 +257,13 @@ public sealed class SceneCompositor
         var ew = (float)element.Width;
         var eh = (float)element.Height;
         var isRound = element.ClipShape == ClipShape.Round;
+        
+        // Webcam diagnostic: Log when webcam frames are processed
+        if (element is WebcamSceneConfig)
+        {
+            AppLog.Write($"Compositor webcam: {frame.Width}x{frame.Height}, IsOpaque={frame.IsOpaque}, element={element.GetType().Name}");
+        }
+        
         // All rect-placed elements route through the paste cache (take-9 lesson): the
         // IsOpaque bypass kept re-sampling the webcam (~156k px) EVERY tick even when
         // the device frame had not changed; an opaque source rasterizes to an opaque

Services/MediaCaptureFrameSource.cs

@@ -317,8 +317,16 @@ public sealed class MediaCaptureFrameSource : ICameraFrameSource
                 }
 
                 // MediaCapture video carries no alpha plane — the OS fills 255.
-                FrameAvailable?.Invoke(new VideoFrame(bitmap.PixelWidth, bitmap.PixelHeight, pixels)
-                    { IsOpaque = true, Epoch = ++_epoch });
+                var frame = new VideoFrame(bitmap.PixelWidth, bitmap.PixelHeight, pixels)
+                    { IsOpaque = true, Epoch = ++_epoch };
+                
+                // Diagnostic: Log frame properties for compositor debugging
+                if ((now - _lastAlphaProbe).TotalSeconds >= 5.0)
+                {
+                    AppLog.Write($"Webcam frame emit: {frame.Width}x{frame.Height}, IsOpaque={frame.IsOpaque}, Epoch={frame.Epoch}");
+                }
+                
+                FrameAvailable?.Invoke(frame);
             }
         }
         catch (Exception ex)