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/PhysicsTorso.cs

@@ -92,6 +92,9 @@ public partial class PhysicsTorso : Node
             _mesh.Visible = true;
         }
 
+        if (BodyMath.ResetIfNonFinite(_body, new Transform3D(_torsoBasis, target), "PhysicsTorso"))
+            return;
+
         if (_body.GlobalPosition.DistanceTo(target) > SnapDistance)
         {
             _body.GlobalTransform = new Transform3D(_torsoBasis, target);
@@ -109,13 +112,9 @@ public partial class PhysicsTorso : Node
             force = force.Normalized() * MaxForce;
         _body.LinearVelocity += force / TorsoMass * dt;
 
-        Quaternion qErr = _torsoBasis.GetRotationQuaternion()
-            * _body.GlobalTransform.Basis.GetRotationQuaternion().Inverse();
-        if (qErr.W < 0f)
-            qErr = new Quaternion(-qErr.X, -qErr.Y, -qErr.Z, -qErr.W);
-        float angle = qErr.GetAngle();
-        Vector3 spinAxis = angle > 1e-5f ? qErr.GetAxis() : Vector3.Up;
-        Vector3 spinDesired = spinAxis * Mathf.Min(angle * OrientKp, OrientMaxSpin);
+        Vector3 rotErr = BodyMath.RotationError(_torsoBasis.GetRotationQuaternion(),
+            _body.GlobalTransform.Basis.GetRotationQuaternion());
+        Vector3 spinDesired = (rotErr * OrientKp).LimitLength(OrientMaxSpin);
 
         Vector3 wErr = spinDesired - _body.AngularVelocity;
         float maxDeltaW = OrientMaxAlpha * dt;