feat(video): add composite-aware captions and harden B-roll review

[Wandi-Liao]

What changed

• Added composite-aware spatial placement and review evidence for captions.
• Added speaker-inset composite review and presentation-route preservation for B-roll.
• Added source-direct B-roll normalization with fixed delivery encoding.
• Added final-size speaker legibility evidence and fractional ROI motion handling.
• Added regression coverage for normalization, review provenance, and speaker inset behavior.

Why it changed

These changes prevent captions and B-roll overlays from obscuring important visual content, preserve B-roll quality and exact frame timing, and make review decisions traceable across revisions.

How it was checked

• B-roll unit suite: 287 tests passed.
• Source-direct normalization regressions: 4 tests passed.
• Caption style, interaction, review, spatial-context, protocol, structure, Python, JavaScript, and JSON checks were run during development.
• Git diff whitespace checks passed.