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>

AGENTS.md

@@ -215,9 +215,10 @@ device only; the editor has an XR simulator for dev. Trackers/action map: see `x
    **Grab v5.3.1 VALIDATED in-headset (owner: "it just works") — grab saga closed.**
    `PhysicsHands.cs`: physical palm bodies (0.4 kg, 90 N chase, rotation lock, stuck-safety
    snap — SUSPENDED for a hand while it grips, HAND ghosts > 10 cm lag). Collision layers:
-   **1 = world+props, 2 = UI, 3 = player body (PhysicsTorso chest), 4 = player hands** —
-   chest↔own hands excluded BOTH ways (no self-collision, Nimso/VRChat precedent); props
-   mask 1|3|4, floor/table mask 1|3|4, hands 4/1|4, chest 3/1|3; panel stays 2/0 (props pass
+   **1 = world+props, 2 = UI, 3 = player body (PhysicsTorso chest), 4 = player hands,
+   5 = player arms (PhysicsArms segments)** — no self-collision between own body parts
+   (Nimso/VRChat precedent); props/floor/table mask 1|3|4|5, hands 4/1|4, chest 3/1|3,
+   arm segments 5/1; panel stays 2/0 (props pass
    through even when shown); layers live on
    CollisionObject3D, NOT CollisionShape3D. Panel = prop-UI prototype (ax_button summon,
    contact-press knobs, Reset). Laser/distant grab REMOVED (user never knew it existed);
@@ -241,14 +242,16 @@ device only; the editor has an XR simulator for dev. Trackers/action map: see `x
    for servo errors — `BodyMath.RotationError` (atan2) is the rule; every driven body runs
    `BodyMath.ResetIfNonFinite`.** Regression: `scenes/collapse_spike.tscn` (run headless with
    `--fixed-fps 90`) must print "COLLAPSE: overall PASS". `BodyRig.cs` is
-   VISUALS-only (head cube + 2-bone analytic arms, soft IK, elbow pole,
-   reach budget feeding `PhysicsTorso.ReachOffset`). Headless regression:
+   VISUALS-only (head cube + arm boxes drawn through the physical elbow; IK + reach budget
+   moved to `PhysicsArms`, item 10). Headless regression:
    `scenes/body_spike.tscn` must print "BODY: overall PASS" (also proves the Box3D pair
    rules: one-way contact, hand-through-chest exclusion, prop contact) — note its repro gap:
    no static-table overlap, no welded heavy props. Stage 2 (physical
    arm chains + hierarchical per-joint budgets, Nimso/Boneworks tier) — see item 10; design + prior art in `research/vr-body-physics-hugs.md`,
    stack facts in `research/vr-body-physics-godot.md`.
-9. **Getting a body (rig) — placeholder v1.3 (cube character, agent-built).** `BodyRig.cs`:
+9. **Getting a body (rig) — placeholder v1.3 (cube character, agent-built).** (The IK/yaw/reach
+   math described here now lives in `PhysicsArms.cs` / `PhysicsTorso.cs`; `BodyRig.cs` only
+   draws.) Original notes:
    all-BOX primitives (owner preference) — head 0.15³ behind the eyes, slim torso retracted,
    wrist-anchored 2-bone arms. **Soft IK applied**: shoulder→hand distance clamped to
    reach×0.98 before the solve (the straight-arm singularity was the overextension shake; if it
@@ -263,3 +266,18 @@ device only; the editor has an XR simulator for dev. Trackers/action map: see `x
    the head/neck look override exists. **HANDOFF.md at project root is the takeover document**
    (constant table verified against code — code wins on drift); keep it updated when systems
    change.
+10. **Body physics Stage 2 — PHYSICAL ARMS, implemented 2026-10-02, awaiting owner playtest.**
+   `PhysicsArms.cs`: per side an upper-arm + forearm RigidBody3D (2.0 / 1.2 kg, layer 5) that
+   PD-chase their analytic 2-bone IK pose, solved each physics tick from the PHYSICAL chest
+   shoulders to the PHYSICAL hands. **No joints** — Box3D has no usable 3-DOF shoulder/wrist
+   joint (PinJoint's hidden identity spring, no Generic6DOF) and this file's architecture
+   forbids joints between tracked bodies; the research's hinge-motor chain is therefore
+   deferred, chase segments are the Box3D-honest version. Hierarchical budgets (Nimso muscle
+   model) from a 60 kg body-mass stat: hand 90 N ≺ forearm 180 N ≺ upper arm 360 N ≺ chest
+   2400 N. What it gives: arms that push props, rest on / get blocked by geometry (no
+   tunneling), and draw honestly (BodyRig's elbow = the physical elbow). What it does NOT do
+   yet (Stage 2b candidates, owner feel decides): a blocked forearm does not hold the HAND
+   back (hands still chase the controllers independently), and prop/NPC forces on the arm
+   don't flow into the chest. Regression: `scenes/arm_spike.tscn` "ARM: overall PASS";
+   the full body also runs inside `scenes/collapse_spike.tscn`. Design + prior art:
+   `research/vr-body-physics-hugs.md` §5–7, `research/vr-body-physics-godot.md`.