Two follow-ups from the creator's lip-sync observation.
1. Audio still slept a fixed 50 ms per chunk — the same "sleep on top of the work" fault fixed for
video an hour ago, left in place. Every chunk's read cost was added, so a clip's audio fell
progressively behind its picture: small per chunk, unbounded over a long clip. It now uses the
same deadline pacer, so audio and video advance on the same wall clock.
2. The preview pane is NOT the compositor. "Preview looks great" proves the decode and the preview
bitmap; the recording and stream are blitted by SceneCompositor. `MediaCompositeOutputTests`
renders a keyed video layer over a background through the real compositor and asserts the two
things that matter: keyed (alpha 0) pixels let the background through, and the subject half
covers it. So "green screen removed" is now proven in the output path, not just the pane.
Also: a round-clipped video layer had no preview trigger (only webcam+Round existed), so it would
render in the output and be invisible in the pane. Same unit-circle path now covers it.
Tests: 16 scoped (pacer, real-clip rate + keying, composite output, decode pacing, alert pacing)
green. FULL SUITE NOT RUN — the machine is in use: two runs under load aborted in the WPF test host
(HWND subclass crash) and one hit the 15-minute cap. Re-run scripts/verify.sh in a quiet window.
