feat(subtitle-studio): migrate plugin to registry

[superche]

Summary

• migrate the complete Subtitle Studio UI, subtitle document/SRT/translation logic, MCP companion, and native hard-subtitle eraser source into convax-plugins
• consume only generic Convax v3 capabilities for connected media, generation workspaces, text-only Agent prompts, text export, and portable WebVTT tracks
• bind persisted subtitle documents to sourceVersion, automatically extract the video audio track, support soft/hard erasure, and mux/publish/export the active multilingual subtitle tracks

Dependency

Requires https://github.com/microvoid/convax/pull/20.

Verification

• bun run check
• validated 16 packages in sequence 19
• 220 tests / 1045 expectations
• Subtitle Studio companion: 36 tests / 129 assertions
• package builds, packs, registry index, and browser bundle passed

Release gate

This PR intentionally remains Draft. The current Registry companion contract installs one bare executable up to 128 MiB, while Subtitle Studio needs a verified sibling runtime tree: ffmpeg/ffprobe, Whisper executables and models, OCR/LaMa assets, and the native eraser. The companion fails closed when that tree is incomplete, and the publish workflow blocks plugin-subtitle-studio-v* tags. Do not publish a release until the runtime distribution contract or a complete pinned runtime bundle lands.