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/PropShop.cs

@@ -45,7 +45,7 @@ public partial class PropShop : Node3D
     {
         Color wood = new(0.45f, 0.3f, 0.18f);
         StaticBody3D table = new();
-        table.CollisionMask = 0b1101;  // world + player body + hands
+        table.CollisionMask = 0b11101;  // world + player body + hands + arms
         table.AddChild(new CollisionShape3D
         {
             Position = new Vector3(0, 0.725f, -0.6f),
@@ -96,7 +96,7 @@ public partial class PropShop : Node3D
             ContinuousCd = spec.Mass < 0.1f,
             ContactMonitor = true,
             MaxContactsReported = 4,
-            CollisionMask = 0b1101,  // world + player body + hands
+            CollisionMask = 0b11101,  // world + player body + hands + arms
         };
         body.AddToGroup("prop");