Body step 1: one-time T-pose calibration (prefs.json, forced gate) + True Body solver + shared ArmSolver

- BodyProfile (Winter anthropometry, T-pose: arm = (span - shoulder width)/2 - 1.5 cm) and
  Prefs (gitignored prefs.json next to the executable / project folder in editor builds)
- Calibration node: loads prefs, else head-locked prompt + jitter-proof TPoseDetector; body
  systems wait for BodyProfile.Current; Recalibrate() in group "calibration"; recal rebuilds arms
- ArmSolver: PhysicsArms' elbow solve extracted (behaviour-identical) + min reach |U-F| fix
- TrueBody: derived real-body pose (neck, chest yaw + crouch lean at neck pivot, lagging pelvis,
  moving shoulder girdle with scapulohumeral shrug, arms) + debug skeleton
- PhysicsArms/PhysicsTorso/BodyRig now profile-driven (arm length no longer from head height)
- TrueBodySpike PASS x3; all spikes PASS x3 (arm spike thresholds re-based for correct 0.57 m arms;
  CollapseSpike uses a fixed standing profile)

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

scripts/PhysicsTorso.cs

@@ -6,7 +6,6 @@ public partial class PhysicsTorso : Node
     private const float MaxForce = 2400f;
     private const float MaxSpeed = 6f;
     public const float SnapDistance = 2f;
-    private const float TorsoDrop = 0.42f;
     private const float TorsoBackOffset = 0.06f;
     private const float HeadBackOffset = 0.06f;
     public const float YawHandsWeight = 0.6f;
@@ -82,12 +81,20 @@ public partial class PhysicsTorso : Node
         Node3D head = HeadSource != null ? HeadSource : (Node3D)_camera;
         if (head == null || _body == null)
             return;
+        // Main scene: wait for the calibrated profile (the T-pose gate). Headless spikes inject
+        // HeadSource and get the anthropometric estimate from their head height instead.
+        BodyProfile profile = BodyProfile.Current ?? (HeadSource != null ? BodyProfile.Estimate(head.GlobalPosition.Y) : null);
+        if (profile == null)
+            return;
+        // Chest centre sits so that its shoulder line (centre + ShoulderRise) is at the
+        // measured shoulder height below the eyes.
+        float torsoDrop = profile.ShoulderDrop + PhysicsArms.ShoulderRise;
         float dt = (float)delta;
 
         _yawBasis = FlatYawBasis(head.GlobalTransform.Basis);
         Vector3 headPos = head.GlobalPosition - _yawBasis * Vector3.Forward * HeadBackOffset;
         UpdateTorsoYaw(headPos, dt);
-        NominalTarget = headPos + _torsoBasis * new Vector3(0f, -TorsoDrop, TorsoBackOffset);
+        NominalTarget = headPos + _torsoBasis * new Vector3(0f, -torsoDrop, TorsoBackOffset);
         Vector3 target = NominalTarget + ReachOffset;
         TargetPosition = target;