fix(paper2reel): preserve baked bottom bar captions

[Happy (huangyangyu)]

Summary

• Use the final Paper2Video video.mp4 as the Paper2Reel playback source.
• Build full, slide, and section clips from that final video so the baked Bottom Bar remains visible.
• Hide the CC control and omit VTT tracks to prevent duplicate subtitles.
• Update only the Paper2Reel media contract, QA gate, and documentation to match.

Scope

• Paper2Reel only.
• No paper2video changes.
• No test files added or committed.

Verification

• Python compilation passed for the four modified scripts.
• git diff --check passed.
• Historical Reel smoke test produced 25 MP4 assets at 1920x1166 with audio.
• Browser verification found the CC control hidden and zero subtitle tracks.
• HTTP Range requests returned 206.

Validated example: https://researchstudio.site/reel/reel_generate/947402ed-8e0d-4724-9d51-537eef597865/2608.16812/reel.html?fix=b45af91