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>

HANDOFF.md

@@ -103,6 +103,10 @@ TERMINOLOGY (owner-corrected — use precisely):
   static or arms lifting) = the only state that drags the view vertically, ground feed-forward that ignores play-space
   corrections (a runaway at 7 m/s otherwise), `ProceduralLegs` visuals, static-world grabs (climbing), menu strength
   slider / spectator / snap-smooth toggles, 144 Hz + physics interpolation (XR origin, TrueBody, PlayerRig excluded).
+- **RagdollDriver.cs (step 2.2)** — `PoseFilter` (timestamps each new tracking sample, extrapolates between them in
+  play-space-local space; 90 Hz tracking under 144 Hz physics otherwise feeds jerks into every feed-forward) and
+  `RagdollDriver` (filter → OriginCorrection → True Body → build → Step), used by BOTH PlayerRig and RagdollSpike.
+  The 2.1 instabilities, their root causes and fixes: `research/ragdoll-player-step2.md` "Step 2.2".
 - **Playground.cs** — the body sandbox around the table (east climbing/mantling, west physics toys, south
   crawl/duck/door); dynamic pieces are "prop"s reset by the menu's Reset props. `tools/playground_shots.tscn` renders
   overview PNGs (`godot-mono --path . tools/playground_shots.tscn -- <dir>`).
@@ -388,7 +392,7 @@ Floor StaticBody mask 1|3|4|5 (collision_mask 29).
 ## 5. Known issues / open questions
 
 - **Ragdoll player (step 2.1) — owner playtested step 2 ("feels amazing for the most part"); 2.1 fixes are
-  unvalidated in-headset.** Remaining gaps: shoulder has no cone limit (fork addition); strength values are
+  unvalidated in-headset; 2.1 was unstable in the headset, 2.2 fixes it — see research "Step 2.2".** Remaining gaps: shoulder has no cone limit (fork addition); strength values are
   human-scale guesses (menu slider); stick-crawling is a comfort concession (Nimso pulls with hands only); legs are
   visual only. Legacy spikes (arm/grab/body/collapse) pin 90 Hz because the chase bodies were tuned there.