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.

ytLive.Tests/AudioSyncDelayTests.cs

@@ -47,6 +47,44 @@ public class AudioSyncDelayTests
         Assert.Equal(960, MarkerShift(delay, 480 * 2, configureMs: 10));
     }
 
+    /// <summary>2026-09-12 total-silence regression: <see cref="AudioMixer.FillAndMix"/>
+    /// calls <c>Configure(_syncOffsetMs?.Invoke() ?? 0)</c> on EVERY ~10ms tick (it
+    /// re-reads the live UI setting each mix, not just on change) — the old
+    /// `Configure` unconditionally reallocated and zeroed the delay line and reset
+    /// `_writePos` every single call, discarding the buffered marker before the
+    /// delay offset could ever read it back. Any persisted non-zero
+    /// `Audio.SyncOffsetMs` (this repo's default is 0, but TASK 22 testing/a user
+    /// slider left 500 saved) turned the WHOLE recording into digital silence:
+    /// mic/loop level meters (sampled upstream of this stage) stayed honest while
+    /// the delay line fed the encoder pipe pure zeros forever. This test
+    /// reconfigures with the SAME delay before every Process call — exactly the
+    /// mixer's calling pattern — and the marker must still survive.</summary>
+    [Fact]
+    public void RepeatedConfigureWithSameDelay_EveryTick_StillDelivers_TheMarker()
+    {
+        const int chunkLen = 960; // 10ms of 48kHz interleaved stereo
+        var delay = new AudioSyncDelay(48000);
+        var chunk = new float[chunkLen];
+        var output = new float[chunkLen];
+        chunk[0] = 1f; // sole non-zero: left of frame 0
+
+        var foundAt = -1;
+        var total = 0;
+        for (var c = 0; c < 6 && foundAt < 0; c++)
+        {
+            delay.Configure(10); // the mixer calls this every tick, not just on change
+            delay.Process(chunk, output);
+            for (var i = 0; i < chunkLen; i++)
+            {
+                if (output[i] != 0f) { foundAt = total + i; break; }
+            }
+            total += chunkLen;
+            chunk[0] = 0f; // only the first tick carries the marker
+        }
+
+        Assert.Equal(960, foundAt); // same 10ms-delay shift as the single-Configure test
+    }
+
     private static int MarkerShift(AudioSyncDelay delay, int chunkLen, int configureMs = -1)
     {
         if (configureMs >= 0)