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/PhysicsHands.cs

@@ -88,6 +88,8 @@ public partial class PhysicsHands : Node
     {
         if (body == null)
             return;
+        if (BodyMath.ResetIfNonFinite(body, controller.GlobalTransform, "PhysicsHands"))
+            return;
         int hand = _left == controller ? 0 : 1;
         Vector3 target = controller.GlobalPosition;
 
@@ -121,18 +123,21 @@ public partial class PhysicsHands : Node
                 return;
             }
         }
-        Vector3 desiredVelocity = (target - body.GlobalPosition) / delta;
+        Drive(body, controller.GlobalTransform, delta);
+    }
+
+    // The hand chase itself — public so headless spikes drive their stand-ins with the real code.
+    public static void Drive(RigidBody3D body, Transform3D target, float delta)
+    {
+        Vector3 desiredVelocity = (target.Origin - body.GlobalPosition) / delta;
         Vector3 dv = desiredVelocity - body.LinearVelocity;
         Vector3 force = dv / delta * body.Mass;
         if (force.Length() > HandMaxForce)
             force = force.Normalized() * HandMaxForce;
         body.LinearVelocity += force / body.Mass * delta;
 
-        Basis deltaBasis = controller.GlobalTransform.Basis * body.GlobalTransform.Basis.Inverse();
-        Quaternion q = deltaBasis.GetRotationQuaternion();
-        Vector3 spin = q.GetAxis() * (q.GetAngle() / delta);
-        if (spin.Length() > HandMaxSpin)
-            spin = spin.Normalized() * HandMaxSpin;
-        body.AngularVelocity = spin;
+        Vector3 rotErr = BodyMath.RotationError(target.Basis.GetRotationQuaternion(),
+            body.GlobalTransform.Basis.GetRotationQuaternion());
+        body.AngularVelocity = (rotErr / delta).LimitLength(HandMaxSpin);
     }
 }