Body physics Stage 1: PhysicsTorso chase chest (30 kg PD + yaw servo), collision layers reworked (3=body, 4=hands, no self-collision), BodyRig visual-only, BodySpike regression (PASS 3x, all spikes green)

scripts/PhysicsHands.cs

@@ -66,8 +66,8 @@ public partial class PhysicsHands : Node
             ContinuousCd = true,
             ContactMonitor = true,
             MaxContactsReported = 8,
-            CollisionLayer = 1,
-            CollisionMask = 1,
+            CollisionLayer = 0b1000,  // layer 4 = player hands
+            CollisionMask = 0b1001,   // world + hands; chest (layer 3) excluded both ways
         };
         body.AddChild(new CollisionShape3D { Shape = new BoxShape3D { Size = HandSize } });
         MeshInstance3D mesh = new() { Mesh = new BoxMesh { Size = HandSize } };