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/Audio/AudioSyncDelay.cs

@@ -29,11 +29,20 @@ public sealed class AudioSyncDelay
         Configure(0);
     }
 
-    /// <summary>Sets the delay in whole milliseconds (clamped to 0..500 and
-    /// flushed).</summary>
+    /// <summary>Sets the delay in whole milliseconds (clamped to 0..500). A no-op
+    /// when the delay hasn't actually changed — <see cref="AudioMixer.FillAndMix"/>
+    /// calls this every ~10ms tick (it re-reads the live UI setting each mix), and
+    /// unconditionally reallocating+zeroing the line every tick (the 2026-09-12
+    /// total-silence bug: any non-zero saved offset wiped the ring before its own
+    /// delayed samples could ever be read back — mic/loop meters, sampled BEFORE
+    /// this stage, kept showing real levels while the pipe stayed pure silence)
+    /// discarded the entire buffered history every 10ms. Changing the delay still
+    /// flushes the line (a live slider change clicks rather than smearing).</summary>
     public void Configure(int delayMs)
     {
-        _delaySamples = Frames(delayMs);
+        var delaySamples = Frames(delayMs);
+        if (delaySamples == _delaySamples && _buffer.Length > 0) return;
+        _delaySamples = delaySamples;
         _buffer = new float[(_delaySamples + _sampleRate) * 2 + 1024];
         _writePos = 0;
     }