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>

scenes/collapse_spike.tscn

@@ -0,0 +1,6 @@
+[gd_scene load_steps=2 format=3]
+
+[ext_resource type="Script" path="res://scripts/CollapseSpike.cs" id="1_spike"]
+
+[node name="CollapseSpike" type="Node3D"]
+script = ExtResource("1_spike")