Stabilize Bone Animation Layer Collapse

🔍 Problem
After editing on a higher bone-animation layer, Collapse can warp or break the original bone animation.
As a result, layers cannot be cleaned up normally and must be kept forever.

💡 Requested feature
Fix bone-animation layer bugs so Collapse keeps pose, timing, and curves identical to before.

🎯 Expected benefit
Smoother layer-based animation edits and more efficient late cleanup/handoff