feat: preview clip framing and captions faithfully

[FernandoAbishai]

Summary

• Add ephemeral draft-specific clip presentation state without changing project export defaults or schema.
• Show real source, 9:16, and 1:1 composition with export-aligned object-fit/object-position framing.
• Render live transcript captions with deletion/caption-hidden filtering, words-per-line chunking, style mapping, karaoke/pop/static behavior, sidecar honesty, and /system/checks fallback messaging.
• Add focused frontend and existing ASS caption contract smokes.

Validation

• npm run smoke:backend — 93 tests passed.
• Frontend clip workspace/draft/publishing/playback/transcript smokes passed.
• npm run lint and npm run build:frontend passed.
• python -m compileall -q backend passed.
• Runtime contract and renderer policy smokes passed.
• Manual visual qualification/export comparison: NOT RUN; no suitable media fixture is present in the repository.

Scope guard

No discovery, review decisions, publishing-copy generation, Export All, face tracking, background/audio preview, backend production export, FFmpeg, runtime, packaging, release, tag, or schema changes.

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