Program render stalls ~100 ms when a take opens a media clip (4-6 delivery slots with no frame) #724

Found during #703 / #723 (2026-09-30).

When a take brings a media clip on air at stream start, Program rendering stops for about 100 ms. The program buffer logs 4–6 consecutive stage=source misses (e.g. delivery slots 26–31), and the next rendered frame lands 4–6 slots later than its frame number.

• A/V impact: fixed separately in #538 Stamp stream video by delivery slot, not render frame number #723. Stream video is now stamped by delivery slot, so the stall is a PTS gap, not an A/V offset.
• Remaining impact: a visible ~100 ms freeze on Program and every output, and 4–6 missing frames. That fails the 60 fps per-frame delivery requirement in CLAUDE.md.

Repro:

• Run scripts/qa/program-buffer-recorded-av.mjs (flash-beep media take right after core launch), or the Stream confidence: measure decoded YouTube A/V sync against local monitoring #703 harness --local-standin.
• The stall shows in about half of sessions, around frames 20–35.

Next step: find where the render thread blocks when the media source opens (MediaTransports apply / first decoder frame / texture creation). The likely fixes are to move that work off the render thread, or hold the last frame without losing slots.