Review follow-up on the salvaged AAC + markdown changes:
- Fix an inaccurate comment claiming the STT layer has a sniff-and-remux
fallback (verified: no such fallback exists; the ffmpeg-absent path caches
raw ADTS and STT may reject it).
- Type the _markdown_to_signal wrapper as tuple[str, list[str]] to match the
shared helper instead of a bare tuple.
- Replace the hardcoded /home/pi/... test fixture with a runtime-generated
ADTS AAC sample so the remux round-trip actually runs in CI (skips only
when ffmpeg is absent) instead of always-skipping.
kshitijk4poor
abafba0762