Arms playtest-2 fixes: flip-proof elbow (angle clamp + rate limit, controller-driven), Stage 2b hand coupling (blocked arm holds the hand back)

- Elbow stored as an angle around the arm axis vs a torso-frame reference: 85% hand-suggested,
  from the controller pose (not the physics palm), clamped +-110 deg, 6 rad/s rate limit
  (flip spike: 1.8 cm/tick max vs 13.8 unclamped)
- PhysicsArms.ConstrainHand: while an arm segment is in contact the hand may lead the
  forearm end by 3 cm; PhysicsHands applies it and suspends stuck-snap while active
- ArmSpike: forearm-rest-over-anvil (owner pose: collider fine, drawn forearm 3.9 cm -> 0),
  flip sweep, bar test now asserts the hand is held back; all six spikes PASS x3

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

scripts/PhysicsHands.cs

@@ -15,6 +15,7 @@ public partial class PhysicsHands : Node
 
     private XRController3D _left;
     private XRController3D _right;
+    private PhysicsArms _arms;
     private readonly RigidBody3D[] _bodies = new RigidBody3D[2];
     private readonly MeshInstance3D[] _ghosts = new MeshInstance3D[2];
     private readonly float[] _stuckTimer = new float[2];
@@ -24,6 +25,7 @@ public partial class PhysicsHands : Node
         _instance = this;
         _left = GetNode<XRController3D>("../XROrigin3D/LeftHand");
         _right = GetNode<XRController3D>("../XROrigin3D/RightHand");
+        _arms = GetNodeOrNull<PhysicsArms>("../PhysicsArms");
         _bodies[0] = MakeHand("LeftHandBody", new Color(0.9f, 0.3f, 0.35f));
         _bodies[1] = MakeHand("RightHandBody", new Color(0.3f, 0.5f, 0.9f));
         _ghosts[0] = MakeGhost();
@@ -92,6 +94,8 @@ public partial class PhysicsHands : Node
             return;
         int hand = _left == controller ? 0 : 1;
         Vector3 target = controller.GlobalPosition;
+        Transform3D driveTarget = controller.GlobalTransform;
+        bool armHeld = _arms != null && _arms.ConstrainHand(hand, ref driveTarget);
 
         bool lagging = body.GlobalPosition.DistanceTo(target) > GhostLagDistance;
         if (_ghosts[hand] != null)
@@ -103,7 +107,9 @@ public partial class PhysicsHands : Node
 
         if (!PropGrabber.IsGrabbing(controller))
         {
-            if (body.GlobalPosition.DistanceTo(target) > StuckDistance)
+            // A hand held back by its own blocked arm is not stuck — snapping it into the
+            // obstacle would just loop.
+            if (body.GlobalPosition.DistanceTo(target) > StuckDistance && !armHeld)
                 _stuckTimer[hand] += delta;
             else
                 _stuckTimer[hand] = 0f;
@@ -123,7 +129,7 @@ public partial class PhysicsHands : Node
                 return;
             }
         }
-        Drive(body, controller.GlobalTransform, delta);
+        Drive(body, driveTarget, delta);
     }
 
     // The hand chase itself — public so headless spikes drive their stand-ins with the real code.