Body step 2.2: fix the 2.1 headset instabilities

Spike now runs real input (90 Hz sample-and-hold tracking + jitter under 144 Hz
physics), which reproduced the playtest. Root causes fixed:
- PoseFilter/RagdollDriver: timestamped extrapolation of tracking, one tick
  pipeline shared by PlayerRig and the spike
- low-pass all feed-forwards (ground control amplified jitter by body mass)
- arm muscles back to local targets (physical-frame aim fed chest rotation back)
- structural hand strength only when braced or holding
- no free-path view chasing while grounded (8 m crouch slide); stick hold-back
- explicit inertia for the Legs body (shape-less tuck tumbled the pelvis)
- world grab: Box3D shape queries return a null collider, use collider_id
- new crouch-hold phases; world-grab detection asserted

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

scripts/PlayerRig.cs

@@ -33,9 +33,9 @@ public partial class PlayerRig : Node
     private bool _haveLastOrigin;
     private Vector3 _moveVelocity;
     private bool _snapArmed = true;
-    private readonly Vector3[] _ctrlPrev = new Vector3[2];
     private readonly MeshInstance3D[] _ghosts = new MeshInstance3D[2];
     private bool _rebuild = true;
+    private RagdollDriver _driver;
 
     public PlayerRagdoll Ragdoll => _ragdoll;
     public static bool Spectator { get; private set; }
@@ -84,6 +84,7 @@ public partial class PlayerRig : Node
         // frame); the body is simulated, so it interpolates.
         _ragdoll = new PlayerRagdoll { Name = "Ragdoll", PhysicsInterpolationMode = PhysicsInterpolationModeEnum.On };
         AddChild(_ragdoll);
+        _driver = new RagdollDriver { Solver = Solver, Ragdoll = _ragdoll };
         BodyProfile.Changed += _ => _rebuild = true;
         for (int i = 0; i < 2; i++)
         {
@@ -121,30 +122,16 @@ public partial class PlayerRig : Node
         if (_haveLastOrigin && _ragdoll.Built && IsJump(_lastOrigin, origin))
             Carry(origin * _lastOrigin.AffineInverse());
 
-        if (_ragdoll.Built && !_rebuild)
-        {
-            // 2. Stick locomotion, 3. reconciliation.
+        // 2. Stick locomotion.
+        if (_ragdoll.Built && !_driver.RebuildRequested)
             Locomote(dt);
-            Vector3 correction = _ragdoll.OriginCorrection(_camera.GlobalPosition, _origin.GlobalPosition.Y, dt);
-            if (correction != Vector3.Zero)
-                _origin.GlobalPosition += correction;
-        }
 
-        // 4. Pose + muscles.
-        Transform3D head = _camera.GlobalTransform;
-        Transform3D left = _left.GlobalTransform, right = _right.GlobalTransform;
-        Solver.Solve(profile, head, left, right, _origin.GlobalPosition.Y, dt);
-        if (_rebuild || !_ragdoll.Built)
-        {
-            _ragdoll.Build(profile, Solver, left, right);
-            _rebuild = false;
-            _ctrlPrev[0] = left.Origin;
-            _ctrlPrev[1] = right.Origin;
-        }
-        Vector3 lv = (left.Origin - _ctrlPrev[0]) / dt, rv = (right.Origin - _ctrlPrev[1]) / dt;
-        _ctrlPrev[0] = left.Origin;
-        _ctrlPrev[1] = right.Origin;
-        _ragdoll.Step(Solver, left, right, lv, rv, _origin.GlobalPosition.Y, dt);
+        // 3. Filtered tracking → play-space correction → True Body → muscles (shared with RagdollSpike).
+        _driver.RebuildRequested |= _rebuild;
+        _rebuild = false;
+        Transform3D o = _origin.GlobalTransform;
+        _driver.Tick(profile, ref o, _camera.Transform, _left.Transform, _right.Transform, _moveVelocity, dt);
+        _origin.GlobalTransform = o;
         _lastOrigin = _origin.GlobalTransform;
         _haveLastOrigin = true;
     }
@@ -162,6 +149,14 @@ public partial class PlayerRig : Node
         }
     }
 
+    // The body is held back (hips against a table, a wall the head clears): stop the stick from
+    // carrying the view further that way.
+    public static Vector3 HoldBack(Vector3 velocity, Vector3 heldBack)
+    {
+        float into = velocity.Dot(heldBack);
+        return into > 0f ? velocity - heldBack * into : velocity;
+    }
+
     private static bool IsJump(Transform3D a, Transform3D b) =>
         a.Origin.DistanceTo(b.Origin) > JumpDistance ||
         BodyMath.RotationError(b.Basis.GetRotationQuaternion(), a.Basis.GetRotationQuaternion()).Length() > JumpAngle;
@@ -170,8 +165,6 @@ public partial class PlayerRig : Node
     {
         _ragdoll.TeleportBy(delta);
         Solver.Shift(delta);
-        _ctrlPrev[0] = delta * _ctrlPrev[0];
-        _ctrlPrev[1] = delta * _ctrlPrev[1];
     }
 
     private void Locomote(float dt)
@@ -195,6 +188,7 @@ public partial class PlayerRig : Node
         if (!_ragdoll.Grounded)
             desired = _moveVelocity;  // no air control
         _moveVelocity = _moveVelocity.MoveToward(desired, MoveAcceleration * dt);
+        _moveVelocity = HoldBack(_moveVelocity, _ragdoll.HeldBack);
         if (_moveVelocity != Vector3.Zero)
             _origin.GlobalPosition += _moveVelocity * dt;