Fix split clip losing keyframe interpolation on right half

[PriyeshPandey2000]

When splitting a clip between two keyframes, splitKeyframeTrack inserts a boundary keyframe at offset 0 of the right half. That keyframe defaulted to .smooth interpolation regardless of the preceding segment. A .hold segment incorrectly animates on the right half; a .linear segment incorrectly eases instead of continuing linearly. Fix looks up the preceding keyframe's interpolationOut and passes it to the inserted boundary keyframe.

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Note

Low Risk
Small, localized change to split keyframe logic with new unit tests; no auth, persistence, or API surface changes.

Overview
Fixes incorrect animation on the right half of a split when the cut falls between keyframes. splitKeyframeTrack still inserts a boundary keyframe at frame 0 on the right track, but that keyframe now uses the interpolationOut of the last keyframe before the split (falling back to .smooth) instead of always defaulting to smooth easing.

That keeps hold segments flat until the next keyframe and linear ramps continuing at a constant rate after the cut. Regression tests cover opacity (hold) and rotation (linear) via splitClip + sample(at:).

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