fix(desktop-gpui): retime mask keyframes when clip speed changes

[yashranaway]

changing a clip's speed moved and resized mask segments but left their keyframes where they were. keyframes are timed from the mask's start, so at half speed a mask animation finished twice as early, and at 2x it ran past the end of the mask.

now each keyframe goes through the same time mapping as the mask edges, so a mask that runs across into the next clip still lines up. the test covers slowing down, speeding up, a mask spanning two clips, and undo/redo.

🤖 Generated with Claude Code

Confidence Score: 5/5

The PR appears safe to merge; the earlier mask drift issue is addressed.

Summary

Preserves mask animation when clip speed changes.

• Mask animations now follow clip speed changes.

The earlier drift finding is addressed: each track now gets an interpolated key at the boundary before its times change.

_{Reviews (2) · Last reviewed commit: "fix(desktop-gpui): split mask keyframes ..." · Reviewed by Greptile}

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[yashranaway]

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[yashranaway]

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