Implement keyframe interpolation in Spriter runtime

[lucidspriter]

Summary

• add per-animation caching helpers to derive current mainline keys and interpolate timeline states every tick
• update setAnimation/tick to reuse the resolved mainline state and maintain a reusable frame-state cache
• mark the phase 3 checklist item for keyframe interpolation as complete now that runtime state updates are in place

Testing

• node --check scml2/c3runtime/instance.js

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Codex Task