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.

MyMistakes.md

@@ -116,6 +116,32 @@ exactly the bug encoded: 50%-blue over red reads `(0,0,128)` instead of `(127,0,
 backdrop never shows through. Next: verify take (element rect shows the backdrop behind
 the widget art), then push gate #1 clears.
 
+**2026-09-12 → COLLATERAL — the SAME transparency saga broke the webcam next.**
+Verify-take of the transparency fix showed a perfect flat gray rectangle where the webcam
+should be (preview fine, recording flat — stdev <1 across the whole element, despite the
+raw camera frame handed to the compositor sampling min=0/max=255 one line earlier in the
+pipeline — proven by instrumenting BOTH ends before touching any code, not guessing).
+Root cause: `ed9d7c1` ("CropBounds metadata + Fill-style scaling... widget fills element
+rect" — take-22 of the SAME transparency saga above) added `cbX/cbY/cbW/cbH` to
+`BlitContentRaw`'s general sampler but only assigned them inside the
+`src.CropBounds is {} cb` branch; every CropBounds-LESS source (webcam, images — anything
+but the web widget) fell through with `cbW=cbH=0`. Since `sxCrop = sxNorm * cbW` and
+`syCrop = syNorm * cbH`, both were always 0, so `sxCanvas`/`syCanvas` collapsed to
+`cbX`/`cbY` = (0,0) for every destination pixel — an entire scaled webcam sampled ONE
+source corner pixel. The web widget (the only CropBounds-bearing source) was never
+affected, which is exactly why the transparency fix's own test suite stayed green while
+this broke. **FIX:** default `cbW = src.Width`, `cbH = src.Height` (cbX/cbY = 0) before
+the branch, only overridden when CropBounds is actually present. ONE test
+`BlitContentRaw_NoCropBounds_SamplesAcrossFullSource_NotJustOrigin` (two-color split
+source, scaled non-1:1 through the paste cache) fails on the old code — right-half pixel
+reads the left half's color — and passes on the fix; proven both ways with a stash/build/
+revert cycle, not by inspection alone.
+**Lesson:** when a shared low-level sampler gains a new optional code path (CropBounds),
+audit EVERY variable the new branch introduces for a safe default in the branch it did
+NOT touch — an uninitialized-to-zero "crop region" silently means "sample only pixel
+(0,0)", not "no crop." grep for this shape (`var x = 0;` followed by an `if (cond) x = ...`
+with no `else`) whenever a conditional metadata field is added to pixel math.
+
 ### Shrink / re-encode an image for the README (screenshots → small hero image)
 
 Worked out 2026-08-29 (the recipe was NEVER recorded the first time it was done, so