Papermotion ペーパーカット物理アニメーション

Using Papermotion-style constraints, plan a 20s paper-cut scene "First Snow".

Deliver:
1. Layer manifest (JSON): each paper piece mesh, hinge joints, mass, z-order.
2. Physics steps: fixed dt, deterministic solver iterations, no random unless seeded.
3. Character rig poses keyed at 12fps on twos, interpolated render at 24fps.
4. Shell commands: render frames, build contact sheet, mux sfx (footstep crunch).
5. Label which samples in upstream repo are Opus 5.5 vs other models.

Optimize for offline render + reproducible SHA of frame folder.