fix: audio silence (idempotent sync-delay configure), webcam gray block, truncated videos

- AudioSyncDelay.Configure reallocated/zeroed its buffer every ~10ms tick
  (AudioMixer re-reads the UI setting each mix), so any non-zero sync offset
  erased the just-written audio -> total silence. Now early-returns when the
  delay samples are unchanged. Regression test proven both ways.
- SceneCompositor.BlitContentRaw defaulted cbW/cbH=0 when CropBounds is null
  (regression from ed9d7c1) -> webcam blit to an empty rect = gray block.
  Default to src.Width/Height. Regression test proven both ways.
- Truncated recordings: rawvideo mux stamps frames at declared 60fps by
  arrival; a scene whose first layer is dynamic (hidden elements still count)
  kills the bake cache -> full render ~35ms -> ~27fps submitted -> halved
  file length. Static scenes bake once (246ms cold, then <1ms) -> 60fps,
  full-length (probe + 13:53 take, 301/300 per 5s, 12.46s file from 12.3s
  wall). FramePump.ProbeRender names the hot render path on slow frames.

Services/Compositor/SceneCompositor.cs

@@ -258,13 +258,7 @@ 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
@@ -434,10 +428,16 @@ public sealed class SceneCompositor
 
         float scale, ox, oy, invScale;
         float drawnW, drawnH;
+        // No CropBounds means "the whole source canvas is the crop region" — cbW/cbH
+        // MUST default to the full source dims, not 0. A zero cbW/cbH makes every
+        // sxCrop/syCrop below evaluate to 0 regardless of position, collapsing the
+        // entire sampled rect to source pixel (0,0) (the webcam gray-block regression,
+        // 2026-09-12: introduced by the CropBounds Fill-style scaling work the same
+        // day — CropBounds-bearing sources, i.e. the web widget, were never affected).
         var cbX = 0;
         var cbY = 0;
-        var cbW = 0;
-        var cbH = 0;
+        var cbW = src.Width;
+        var cbH = src.Height;
 
         if (src.CropBounds is { } cb)
         {