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>

scenes/main.tscn

@@ -1,4 +1,4 @@
-[gd_scene load_steps=20 format=3]
+[gd_scene load_steps=21 format=3]
 
 [ext_resource type="Script" path="res://scripts/Main.cs" id="1_main"]
 [ext_resource type="PackedScene" path="res://scenes/teleport_marker.tscn" id="2_marker"]
@@ -10,6 +10,7 @@
 [ext_resource type="Script" path="res://scripts/PhysicsHands.cs" id="8_physicshands"]
 [ext_resource type="Script" path="res://scripts/BodyRig.cs" id="9_bodyrig"]
 [ext_resource type="Script" path="res://scripts/PhysicsTorso.cs" id="10_physicsTorso"]
+[ext_resource type="Script" path="res://scripts/PhysicsArms.cs" id="11_physicsArms"]
 
 [sub_resource type="ProceduralSkyMaterial" id="ProceduralSkyMaterial_sky"]
 sky_horizon_color = Color(0.66, 0.72, 0.81, 1)
@@ -56,7 +57,7 @@ shadow_bias = 0.04
 shadow_normal_bias = 0.5
 
 [node name="Floor" type="StaticBody3D" parent="."]
-collision_mask = 13
+collision_mask = 29
 
 [node name="FloorShape" type="CollisionShape3D" parent="Floor"]
 transform = Transform3D(1, 0, 0, 0, 1, 0, 0, 0, 1, 0, -0.05, 0)
@@ -118,6 +119,9 @@ script = ExtResource("8_physicshands")
 [node name="PhysicsTorso" type="Node" parent="."]
 script = ExtResource("10_physicsTorso")
 
+[node name="PhysicsArms" type="Node" parent="."]
+script = ExtResource("11_physicsArms")
+
 [node name="BodyRig" type="Node3D" parent="."]
 script = ExtResource("9_bodyrig")