feat: add alpha probe diagnostics to webcam frame capture

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

Services/MediaCaptureFrameSource.cs

@@ -267,6 +267,9 @@ public sealed class MediaCaptureFrameSource : ICameraFrameSource
     private int _ringNext;
     private long _epoch;
 
+    // Alpha probe diagnostics (one-shot, throttled)
+    private DateTime _lastAlphaProbe = DateTime.MinValue;
+
     private byte[] RentRingBuffer(int size)
     {
         for (var tries = 0; tries < _frameRing.Length; tries++)
@@ -304,6 +307,15 @@ public sealed class MediaCaptureFrameSource : ICameraFrameSource
             {
                 var pixels = RentRingBuffer((int)capacity);
                 Marshal.Copy(pixelsPtr, pixels, 0, (int)capacity);
+
+                // Alpha probe diagnostics (throttled to once per 5s)
+                var now = DateTime.UtcNow;
+                if ((now - _lastAlphaProbe).TotalSeconds >= 5.0)
+                {
+                    _lastAlphaProbe = now;
+                    ProbeAlphaStats(pixels, bitmap.PixelWidth, bitmap.PixelHeight);
+                }
+
                 // MediaCapture video carries no alpha plane — the OS fills 255.
                 FrameAvailable?.Invoke(new VideoFrame(bitmap.PixelWidth, bitmap.PixelHeight, pixels)
                     { IsOpaque = true, Epoch = ++_epoch });
@@ -319,4 +331,37 @@ public sealed class MediaCaptureFrameSource : ICameraFrameSource
                 bitmap.Dispose();
         }
     }
+
+    private void ProbeAlphaStats(byte[] pixels, int width, int height)
+    {
+        try
+        {
+            // Sample 1000 alpha bytes (every 4th byte in BGRA format)
+            var sampleCount = Math.Min(1000, pixels.Length / 4);
+            var alphaMin = 255;
+            var alphaMax = 0;
+            var alphaSum = 0;
+
+            for (var i = 0; i < sampleCount; i++)
+            {
+                var alphaIndex = (i * 4) + 3; // Alpha is 4th byte in BGRA
+                if (alphaIndex < pixels.Length)
+                {
+                    var alpha = pixels[alphaIndex];
+                    alphaMin = Math.Min(alphaMin, alpha);
+                    alphaMax = Math.Max(alphaMax, alpha);
+                    alphaSum += alpha;
+                }
+            }
+
+            var alphaMean = sampleCount > 0 ? alphaSum / (double)sampleCount : 0;
+            AppLog.Write($"Webcam alpha probe: {width}x{height}, epoch {_epoch}, " +
+                         $"alpha[min={alphaMin}, max={alphaMax}, mean={alphaMean:F1}], " +
+                         $"sampled {sampleCount} pixels");
+        }
+        catch (Exception ex)
+        {
+            AppLog.Write($"Webcam alpha probe failed: {ex.Message}");
+        }
+    }
 }