feat: add MCP timeline MP4 rendering

[erickkkyt]

Summary

• add bdesign_editor_render as the 27th BeatDesign MCP tool for authoritative Timeline-to-MP4 rendering
• render visible clips, image overlays, burned-in captions, and mixed audio into a project-owned Asset
• invalidate stale current-render pointers after render-affecting UI or MCP edits
• synchronize existing Canvas Timeline cards with the authoritative Editor render state
• route legacy UI Timeline replacement through the shared Command Kernel
• update English, Chinese, and Japanese docs plus Codex, Claude Code, and WorkBuddy Agent Skills

Test coverage

• regression coverage for stale render invalidation after incremental MCP edits
• regression coverage for UI full-document replacement invalidation
• regression coverage for preserving a newly recorded render
• regression coverage for coupled Canvas render ID/URL clearing
• contract coverage ensuring the legacy Timeline route uses the Command Kernel
• MCP catalog coverage updated to exactly 27 registered tools

Real MCP verification

Using the existing BeatDesign promotion-video project:

1. changed a caption through bdesign_editor_edit
2. confirmed both Editor and Canvas cleared the previous current render
3. restored the caption through MCP
4. exported through bdesign_editor_render
5. confirmed Editor and Canvas both pointed to the same new 15-second project MP4 Asset

The generated file was verified as 1080x1920 H.264 with AAC stereo audio.

Pre-landing review

Two issues were found and fixed before commit:

• separated missing ffmpeg/ffprobe startup errors from unrelated Asset persistence failures
• synchronized stale documentation and Agent instructions with the new MCP capability

No remaining blocking findings.

Security

• repository pre-commit security scanner: clean
• no shell interpolation is used for media-process arguments
• render inputs resolve only through validated project-owned local Asset paths
• pnpm audit --prod --audit-level high: no known vulnerabilities

Verification

• pnpm typecheck
• pnpm test
• pnpm i18n:check
• pnpm build
• git diff --check
• staged security scan for both commits
• real stdio MCP edit/invalidate/render synchronization flow

GitHub CI status is intentionally left to the PR checks after creation.

Current boundary

Browser-native MP4 export still requires no system FFmpeg. MCP/Node rendering requires ffmpeg and ffprobe on PATH, or explicit BEATDESIGN_FFMPEG and BEATDESIGN_FFPROBE paths.

Plan completion

No separate plan file was used; the implementation follows the repository product, architecture, provider, design, and release-scope documents.

TODOs

No repository TODO items were modified.

Documentation

• README.md, README.zh-CN.md, and README.ja.md: updated the public MCP catalog to 27 tools and added authoritative Timeline MP4 rendering.
• docs/MCP.md: documented the render workflow, revision behavior, output Asset, and local ffmpeg/ffprobe requirements.
• docs/PRODUCT_PLAN_AND_STATUS.md: aligned shipped capability and remaining headless-worker boundaries.
• AGENTS.md and third_party/README.md: clarified that browser export remains FFmpeg-free while optional MCP rendering uses user-provided binaries.
• Codex, Claude Code, and WorkBuddy integration Skills: added the MCP export workflow and its authorization/error-handling rules.
• CHANGELOG.md: recorded MCP rendering and stale-render synchronization under Unreleased.