feat: edit opacity keyframes in the media toolbar

[dazzatronus]

What changed

• Add three-state opacity keyframe controls to the media toolbar, with frame-snapped editing and previous/next navigation.
• Serialise linear opacity animation into the existing Tween[] Edit contract. There is no schema or exported API change.
• Keep unsupported Tween shapes read-only and block controls that would overwrite or conflict with keyframed values.

Verification

• npm run verify:ci (69 suites, 1,932 tests)
• npm pack --dry-run --json
• Browser: create, edit, delete and navigate keys; playback; undo/redo; trim/extend; reload

[github-actions]

🎉 This PR is included in version 2.16.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀