Add deliverable variant manager

[RayJiang4S]

Summary

• add a variant_manager project tool for tracking render candidates, approved deliverables, current delivery channels, and variant lineage
• add a variant_manifest schema and documentation for post-compose / publish-prep usage
• register the artifact and document when agents should maintain the manifest

Tests

• .venv/bin/python -m pytest tests/tools/test_variant_manager.py -q
• .venv/bin/python -m pytest tests/contracts/test_phase0_contracts.py -q

Notes

This is intentionally a lightweight ledger. It does not store media bytes, replace review platforms, or auto-judge creative quality; it keeps project deliverable versions answerable after renders begin to branch.

[RayJiang4S]

Added a follow-up refinement to the Variant Manager workflow.

This update turns the variant ledger into a more practical human review loop:

• review now generates an interactive HTML review page for deliverable candidates.
• The page embeds local video previews, enforces one-at-a-time playback, supports language-aware UI labels, and copies a review JSON payload for the agent.
• annotate can now apply that pasted review payload across multiple rounds:
  • approved selections become the current channel variant;
  • revision notes keep the variant as a candidate and return next_operation=revise_variant;
  • “none of these” requests are recorded as a new-variant request.
• Once a variant is approved, the result includes package_inputs with video_path, project_id, variant_id, channel, review notes, and sidecars so a final package helper can consume it directly.

I attached a screenshot of the generic English demo page to show the review UX without using project-specific assets.