chore(editor): pin OpenCut browser WebM export

[sjungwon03]

Summary

• Advance external/opencut to the reviewed Theorvane/OpenCut main merge commit a7a84707c15a42fd12584e1e86e6585905e2702b from fork PR #2.
• Document the standalone web editor's local WebM export and the OpenScene integration boundary.

Surface impact

OpenScene imports only the three existing OpenCut UI controls. The new browser export code is not imported by Electron. OpenScene desktop keeps its FFmpeg pipeline, and mobile editing behavior and shared rules do not change.

Verification

• OpenCut fork PR docs(workflow): require issue branch PR flow #2: AI approval on exact head, web typecheck, 11/11 editor tests, web build, and Ubuntu/Windows/macOS CI passed before merge.
• OpenScene: npm run typecheck, npm test (1542/1542), and npm run build passed on this pointer.
• Mobile: npm run typecheck passed. No development client is attached for a manual screen check.
• Browser: no local browser executable was available for a manual WebM export; fork tests use mocked browser recording and audio APIs.

Closes #462

[sjungwon03-ai]

Review summary

Verdict: Approved

• Reviewed: OpenCut submodule pin, integration boundary documentation, and upstream WebM export merge
• Checks: OpenScene CI checks passed; pinned OpenCut merge CI and editor verification passed