chore(editor): pin OpenCut visual controls and fades

[sjungwon03]

Summary

• Advance external/opencut to the reviewed Theorvane/OpenCut main merge commit 4eb332fcb40ecd63aba83646158e0c1180f8e3bb from fork PR #3.
• Document standalone web editor visual adjustments and overlapping clip fades.

Surface impact

OpenScene still compiles only the three existing fork UI controls. The new fork-only visual model, preview, and WebM compositor are not imported by Electron. OpenScene desktop FFmpeg behavior and mobile shared editing rules remain unchanged.

Verification

• OpenCut fork PR chore: establish open-source community and CI baseline #3: exact-head AI approval, TypeScript check, 13/13 editor tests, Vite 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 visual/export check; fork tests cover overlap math and mocked layered recording.

Closes #464

[sjungwon03-ai]

Review summary

Verdict: Approved

• Reviewed: OpenCut submodule pin and integration boundary documentation
• Checks: CI verify, iOS module/export, Android module, and review-request passed