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>

AGENTS.md

@@ -306,6 +306,12 @@ device only; the editor has an XR simulator for dev. Trackers/action map: see `x
    smooth turn, **physics 144 Hz with physics interpolation** (`Main.PhysicsRate`; XROrigin3D, TrueBody, PlayerRig
    are interpolation-OFF because they move per frame — anything new moved in `_Process` must be excluded too).
    Rule learned: play-space corrections must never feed ground control's feed-forward (runaway feedback).
+   **Step 2.2 (2.1 playtest was unstable — see research doc "Step 2.2"): RagdollSpike now runs REAL input
+   (90 Hz sample-and-hold tracking + jitter under 144 Hz physics); a body test on ideal input proves nothing.**
+   Tick pipeline = `RagdollDriver` (shared by PlayerRig and the spike; `PoseFilter` extrapolates tracking).
+   Rules: low-pass every feed-forward; muscles stay local to the TARGET pose (never the physical parent — rotation
+   feedback); no free-path view chasing while grounded; bodies whose shapes can all be disabled need explicit
+   inertia; Box3D shape queries return a null `collider` — use `collider_id`.
    Owner scope: thumbstick locomotion only, no jump, avatar scaling and NPCs later.
 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