Body physics Stage 2: physical arm segments (chase-driven upper arm + forearm, hierarchical budgets, layer 5)

- PhysicsArms: per side 2.0 kg upper arm + 1.2 kg forearm RigidBody3D PD-chasing their
  analytic 2-bone IK pose, solved per physics tick from the physical chest shoulders to the
  physical hands; no joints (Box3D has no usable shoulder/wrist joint; AGENTS forbids joints
  between tracked bodies). Budgets from a 60 kg mass stat: hand 90 < forearm 180 < upper
  arm 360 < chest 2400 N. Owns calibration + reach budget (moved from BodyRig); 1 m snap,
  non-finite reset, BodyMath.RotationError servo.
- Layer 5 = player arms (mask 1); props/table/floor masks include it; no self-collision.
- BodyRig: visuals only, arms drawn shoulder -> physical elbow -> wrist.
- ArmSpike: track gap 0, swing lag <= 6 cm, arm pushes prop with hand clear, forearm blocked
  by bar without tunneling and quiet when pinned, teleport snap, NaN reset — PASS x3.
- CollapseSpike now runs the full body (49/49 x3); all six spikes PASS x3.
- HANDOFF/AGENTS: Stage 2 entry (item 10), layers, tuning table, Stage 2b next steps.

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

scripts/CollapseSpike.cs

@@ -1,7 +1,8 @@
 using System.Collections.Generic;
 using Godot;
 
-// Headless regression for the Stage-1 playtest collapse (2026-10-01). Two families:
+// Headless regression for the Stage-1 playtest collapse (2026-10-01), now with the full Stage-2
+// body (chest + PhysicsArms + hands). Two families:
 // - Stacking: seated chest overlapping the static table, 50 kg anvil rigid-welded to two
 //   chase hands, the logged grab sequence (L, R two-handed, L release, L re-grab), the real
 //   PropShop with the chest plowing through the props. Did NOT reproduce the NaN — kept as
@@ -39,6 +40,7 @@ public partial class CollapseSpike : Node3D
     private Node3D _world;
     private Node3D _head;
     private PhysicsTorso _torso;
+    private PhysicsArms _arms;
     private readonly RigidBody3D[] _hands = new RigidBody3D[2];
     private readonly Node3D[] _targets = new Node3D[2];
     private readonly ConeTwistJoint3D[] _welds = new ConeTwistJoint3D[2];
@@ -189,6 +191,9 @@ public partial class CollapseSpike : Node3D
             _world.AddChild(_targets[h]);
             _targetBase[h] = handPos;
         }
+
+        _arms = new PhysicsArms { HeadSource = _head, Torso = _torso, HandLeft = _hands[0], HandRight = _hands[1] };
+        _world.AddChild(_arms);
     }
 
     public override void _PhysicsProcess(double delta)
@@ -258,6 +263,15 @@ public partial class CollapseSpike : Node3D
         Check("handR", _hands[1]);
         if (_anvil != null)
             Check("anvil", _anvil);
+        for (int side = 0; side < 2; side++)
+        {
+            if (_arms.UpperArm(side) == null)
+                continue;
+            Check($"upper arm {side}", _arms.UpperArm(side));
+            Check($"forearm {side}", _arms.Forearm(side));
+            if (_arms.Forearm(side).LinearVelocity.Length() > RunawaySpeed || _arms.UpperArm(side).LinearVelocity.Length() > RunawaySpeed)
+                Fail($"arm {side} runaway");
+        }
         foreach (Node n in GetTree().GetNodesInGroup("prop"))
         {
             if (n is not RigidBody3D prop || prop.IsQueuedForDeletion())