Body Stage 1 collapse root-caused + fixed: NaN in torso servo math (GetAngle = 2*acos(W) at W=1.0000001), not solver stacking

- BodyMath.RotationError: atan2-based shortest-arc error (no NaN, no GetAxis gain collapse
  inside 0.16 deg); used by PhysicsTorso orientation servo and PhysicsHands rotation lock
- BodyMath.ResetIfNonFinite escape hatch on torso + hands
- PhysicsHands.Drive extracted (static) so spikes run the real hand chase
- CollapseSpike: deterministic repro (slow yaw drift across a parked chest, 7/40 NaN on old
  code) + solver-stacking scenarios (seated chest in table, two-hand-welded 50 kg anvil,
  real PropShop) — 49/49 clean after fix, x3; all other spikes PASS x3
- Failure brief RESOLUTION section; HANDOFF/AGENTS updated

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/CollapseSpike.cs.uid

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+uid://cecdf0w5cdovg