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/ArmSpike.cs

@@ -0,0 +1,277 @@
+using System.Collections.Generic;
+using Godot;
+
+// Headless regression for body-physics Stage 2 (PhysicsArms): chase-driven arm segments.
+// Run: godot-mono --headless --fixed-fps 90 --path . scenes/arm_spike.tscn
+public partial class ArmSpike : Node3D
+{
+    private enum Phase { Settle, Track, Swing, Push, Pin, Teleport, NanGuard, Done }
+
+    private const float GapLimit = 0.04f;
+    private const float SwingGapLimit = 0.12f;
+    private const float ChestDisturbLimit = 0.01f;
+    private const float PushMinDisplacement = 0.05f;
+    private const float PushHandClearance = 0.08f;
+    private const float PinSpeedLimit = 0.3f;
+    private const float PinBarY = 1.22f;
+    private const float PinObstructionMin = 0.01f;
+    private const float TeleportGapLimit = 0.04f;
+    private static readonly Vector3 HandSize = new(0.035f, 0.09f, 0.12f);
+
+    private Node3D _head;
+    private PhysicsTorso _torso;
+    private PhysicsArms _arms;
+    private readonly RigidBody3D[] _hands = new RigidBody3D[2];
+    private readonly Transform3D[] _handTarget = new Transform3D[2];
+    private readonly Vector3[] _handRest = new Vector3[2];
+    private RigidBody3D _box;
+    private Vector3 _boxStart;
+    private StaticBody3D _slab;
+
+    private Phase _phase = Phase.Settle;
+    private float _phaseT;
+    private float _maxGap;
+    private float _maxChestErr;
+    private float _minHandBoxDist;
+    private float _maxPinSpeed;
+    private float _maxPinObstruction;
+    private readonly float[] _pinPrevY = { float.MinValue, float.MinValue };
+    private bool _pinTunneled;
+    private bool _pinFinite = true;
+    private readonly List<string> _names = new();
+    private readonly List<bool> _ok = new();
+
+    public override void _Ready()
+    {
+        StaticBody3D floor = new() { CollisionMask = 0b11101 };
+        floor.AddChild(new CollisionShape3D { Shape = new BoxShape3D { Size = new Vector3(20f, 0.1f, 20f) }, Position = new Vector3(0f, -0.05f, 0f) });
+        AddChild(floor);
+
+        _head = new Node3D { Position = new Vector3(0f, 1.65f, 0f) };
+        AddChild(_head);
+        _torso = new PhysicsTorso { HeadSource = _head };
+        AddChild(_torso);
+        _torso.Body.GlobalPosition = new Vector3(0f, 1.23f, 0.12f);
+
+        for (int side = 0; side < 2; side++)
+        {
+            float x = side == 0 ? -0.2f : 0.2f;
+            _handRest[side] = new Vector3(x, 1.1f, -0.35f);
+            _handTarget[side] = new Transform3D(Basis.Identity, _handRest[side]);
+            _hands[side] = new RigidBody3D
+            {
+                Mass = 0.4f,
+                GravityScale = 0f,
+                ContinuousCd = true,
+                Position = _handRest[side],
+                CollisionLayer = 0b1000,
+                CollisionMask = 0b1001,
+            };
+            _hands[side].AddChild(new CollisionShape3D { Shape = new BoxShape3D { Size = HandSize } });
+            AddChild(_hands[side]);
+        }
+
+        _arms = new PhysicsArms { HeadSource = _head, Torso = _torso, HandLeft = _hands[0], HandRight = _hands[1] };
+        AddChild(_arms);
+        GD.Print("ARM: arm spike scene ready");
+    }
+
+    public override void _PhysicsProcess(double delta)
+    {
+        float dt = (float)delta;
+        _phaseT += dt;
+        for (int side = 0; side < 2; side++)
+            PhysicsHands.Drive(_hands[side], _handTarget[side], dt);
+        if (!_arms.Calibrated || _arms.UpperArm(0) == null)
+            return;
+
+        switch (_phase)
+        {
+            case Phase.Settle:
+                if (_phaseT > 1f)
+                    Start(Phase.Track, "track — hands circle, segments must stay connected, chest undisturbed");
+                break;
+
+            case Phase.Track:
+                for (int side = 0; side < 2; side++)
+                {
+                    float w = side == 0 ? 2.1f : 2.7f;
+                    _handTarget[side].Origin = _handRest[side] + new Vector3(0.15f * Mathf.Sin(w * _phaseT), 0.15f * Mathf.Cos(w * _phaseT), 0.1f * Mathf.Sin(0.5f * w * _phaseT));
+                }
+                if (_phaseT > 0.5f)
+                {
+                    _maxGap = Mathf.Max(_maxGap, MaxGap());
+                    _maxChestErr = Mathf.Max(_maxChestErr, _torso.Body.GlobalPosition.DistanceTo(_torso.TargetPosition));
+                }
+                if (_phaseT > 3f)
+                {
+                    Record($"track joint gap max {_maxGap:F3} m (limit {GapLimit})", _maxGap < GapLimit);
+                    Record($"chest undisturbed by own arms (err max {_maxChestErr:F4} m, limit {ChestDisturbLimit})", _maxChestErr < ChestDisturbLimit);
+                    Start(Phase.Swing, "swing — fast 0.4 m arm swings at 2 Hz, bounded lag");
+                }
+                break;
+
+            case Phase.Swing:
+                _handTarget[1].Origin = _handRest[1] + new Vector3(0.2f * Mathf.Sin(12.6f * _phaseT), 0.1f * Mathf.Sin(6.3f * _phaseT), 0f);
+                _maxGap = Mathf.Max(_maxGap, MaxGap());
+                if (_phaseT > 2f)
+                {
+                    _handTarget[1].Origin = _handRest[1];
+                    Record($"swing joint gap max {_maxGap:F3} m (limit {SwingGapLimit})", _maxGap < SwingGapLimit);
+                    Start(Phase.Push, "push — right forearm swings up through a 1 kg box (hand kept clear)");
+                    // Box just above the forearm's middle; the hand rises 0.3 m ~0.2 m away from it.
+                    _boxStart = (_arms.ElbowTarget(1) + _arms.WristTarget(1)) * 0.5f + Vector3.Up * 0.11f;
+                    _box = new RigidBody3D { Mass = 1f, GravityScale = 0f, Position = _boxStart, CollisionMask = 0b11101 };
+                    _box.AddChild(new CollisionShape3D { Shape = new BoxShape3D { Size = Vector3.One * 0.08f } });
+                    AddChild(_box);
+                    _minHandBoxDist = float.MaxValue;
+                }
+                break;
+
+            case Phase.Push:
+            {
+                float k = Mathf.Min(_phaseT / 0.6f, 1f);
+                _handTarget[1].Origin = _handRest[1] + Vector3.Up * (0.3f * k);
+                _minHandBoxDist = Mathf.Min(_minHandBoxDist, _hands[1].GlobalPosition.DistanceTo(_box.GlobalPosition));
+                if (_phaseT > 1.5f)
+                {
+                    float moved = _box.GlobalPosition.DistanceTo(_boxStart);
+                    Record($"arm segment pushes prop (box moved {moved:F3} m, limit > {PushMinDisplacement}; hand-box min dist {_minHandBoxDist:F3} m, limit > {PushHandClearance})",
+                        moved > PushMinDisplacement && _minHandBoxDist > PushHandClearance);
+                    _box.QueueFree();
+                    _handTarget[1].Origin = _handRest[1];
+                    // Two short static bars just above the forearms' middles (x clear of the chest).
+                    // Hands at (±0.2, 1.17, -0.09) put the forearm centers at ~(±0.22, 1.16, 0.09)
+                    // with the elbows swung back; then the hands rise 0.25 m in front of the bars,
+                    // so the forearm targets sweep up through them.
+                    // The bars appear once the forearms have settled in the pin pose (t = 0.8 s).
+                    _maxPinObstruction = 0f;
+                    Start(Phase.Pin, "pin — forearms blocked under static bars while the hands rise past them");
+                }
+                break;
+            }
+
+            case Phase.Pin:
+            {
+                if (_slab == null && _phaseT >= 0.8f)
+                {
+                    _slab = new StaticBody3D { CollisionMask = 0b11101 };
+                    foreach (float x in new[] { -0.22f, 0.22f })
+                        _slab.AddChild(new CollisionShape3D { Position = new Vector3(x, 0f, 0f), Shape = new BoxShape3D { Size = new Vector3(0.14f, 0.04f, 0.04f) } });
+                    AddChild(_slab);
+                    _slab.GlobalPosition = new Vector3(0f, PinBarY, 0.09f);
+                    for (int side = 0; side < 2; side++)
+                        GD.Print($"ARM: pin start side {side}: forearm center {_arms.Forearm(side).GlobalPosition} (bar y {PinBarY})");
+                }
+                float rise = Mathf.Clamp((_phaseT - 1f) / 0.5f, 0f, 1f) * 0.25f;
+                for (int side = 0; side < 2; side++)
+                    _handTarget[side].Origin = new Vector3(side == 0 ? -0.2f : 0.2f, 1.17f + rise, -0.09f);
+                if (_slab != null)
+                {
+                    for (int side = 0; side < 2; side++)
+                    {
+                        RigidBody3D fore = _arms.Forearm(side);
+                        _pinFinite &= BodyMath.IsFinite(fore) && BodyMath.IsFinite(_arms.UpperArm(side));
+                        if (_phaseT > 2.5f)
+                        {
+                            _maxPinSpeed = Mathf.Max(_maxPinSpeed, fore.LinearVelocity.Length());
+                            _maxPinSpeed = Mathf.Max(_maxPinSpeed, _arms.UpperArm(side).LinearVelocity.Length());
+                        }
+                        // Tunneling = the forearm center rises past the bar while still over it in x
+                        // (bar half-width 0.07 + forearm half-width 0.03). Sliding off sideways and
+                        // then rising is honest physics.
+                        Vector3 c = fore.GlobalPosition;
+                        bool overBar = Mathf.Abs(Mathf.Abs(c.X) - 0.22f) < 0.1f;
+                        if (_pinPrevY[side] > float.MinValue && _pinPrevY[side] < PinBarY && c.Y >= PinBarY && overBar)
+                            _pinTunneled = true;
+                        if (_pinPrevY[side] < PinBarY && c.Y >= PinBarY)
+                            GD.Print($"ARM: side {side} forearm cleared the bar at x {c.X:F3} (over bar: {overBar}) t {_phaseT:F2}");
+                        _pinPrevY[side] = c.Y;
+                        Vector3 foreTarget = (_arms.ElbowTarget(side) + _arms.WristTarget(side)) * 0.5f;
+                        _maxPinObstruction = Mathf.Max(_maxPinObstruction, _arms.Forearm(side).GlobalPosition.DistanceTo(foreTarget));
+                    }
+                }
+                if (_phaseT > 3.5f)
+                {
+                    Record($"forearm blocked by bar, no tunneling (deviation max {_maxPinObstruction:F3} m, limit > {PinObstructionMin}; tunneled {_pinTunneled})",
+                        _maxPinObstruction > PinObstructionMin && !_pinTunneled);
+                    Record($"pinned under bar: finite and quiet in the last 1 s (segment |v| max {_maxPinSpeed:F3} m/s, limit {PinSpeedLimit})", _pinFinite && _maxPinSpeed < PinSpeedLimit);
+                    _slab.QueueFree();
+                    for (int side = 0; side < 2; side++)
+                        _handRest[side] += new Vector3(3f, 0f, 0f);
+                    _head.GlobalPosition += new Vector3(3f, 0f, 0f);
+                    for (int side = 0; side < 2; side++)
+                    {
+                        _handTarget[side].Origin = _handRest[side];
+                        _hands[side].GlobalPosition = _handRest[side];
+                        _hands[side].LinearVelocity = Vector3.Zero;
+                    }
+                    Start(Phase.Teleport, "teleport — head + hands jump 3 m, arms must snap");
+                }
+                break;
+            }
+
+            case Phase.Teleport:
+                if (_phaseT > 0.4f)
+                {
+                    float gap = MaxGap();
+                    Record($"teleport re-attach gap {gap:F3} m (limit {TeleportGapLimit})", gap < TeleportGapLimit);
+                    _arms.Forearm(0).AngularVelocity = new Vector3(float.NaN, 0f, 0f);
+                    Start(Phase.NanGuard, "nan guard — inject NaN into the left forearm, it must reset");
+                }
+                break;
+
+            case Phase.NanGuard:
+                if (_phaseT > 0.3f)
+                {
+                    bool finite = BodyMath.IsFinite(_arms.Forearm(0));
+                    float gap = MaxGap();
+                    Record($"non-finite segment reset (finite {finite}, gap {gap:F3} m)", finite && gap < GapLimit);
+                    Finish();
+                }
+                break;
+        }
+    }
+
+    private float MaxGap()
+    {
+        float gap = 0f;
+        for (int side = 0; side < 2; side++)
+        {
+            RigidBody3D upper = _arms.UpperArm(side);
+            RigidBody3D fore = _arms.Forearm(side);
+            Vector3 upperProx = upper.GlobalPosition - upper.GlobalTransform.Basis.Y * (_arms.UpperLen * 0.5f);
+            Vector3 upperDist = upper.GlobalPosition + upper.GlobalTransform.Basis.Y * (_arms.UpperLen * 0.5f);
+            Vector3 foreProx = fore.GlobalPosition - fore.GlobalTransform.Basis.Y * (_arms.ForeLen * 0.5f);
+            gap = Mathf.Max(gap, upperProx.DistanceTo(_arms.Shoulder(side)));
+            gap = Mathf.Max(gap, upperDist.DistanceTo(foreProx));
+            gap = Mathf.Max(gap, _arms.Wrist(side).DistanceTo(_arms.WristTarget(side)));
+        }
+        return gap;
+    }
+
+    private void Start(Phase phase, string what)
+    {
+        _phase = phase;
+        _phaseT = 0f;
+        _maxGap = 0f;
+        GD.Print($"ARM: {what}");
+    }
+
+    private void Record(string name, bool ok)
+    {
+        _names.Add(name);
+        _ok.Add(ok);
+        GD.Print($"ARM: {name} → {(ok ? "PASS" : "FAIL")}");
+    }
+
+    private void Finish()
+    {
+        _phase = Phase.Done;
+        bool overall = true;
+        foreach (bool ok in _ok)
+            overall &= ok;
+        GD.Print($"ARM: overall {(overall ? "PASS — physical arms functional on Box3D" : "CHECK — inspect failures above")}");
+        GetTree().Quit();
+    }
+}