Arms playtest-1 fixes: open-loop torso lean (chest hunting), hand-driven elbow pole + height cap, held-prop arm collision exceptions

- Lean computed from PhysicsTorso.NominalTarget/TargetBasis, not the body's own shoulders:
  the closed loop made the chest hunt (spike: |v| 1.1 -> 0.019 m/s, lean range 2.7 cm -> 0.9 mm)
- Elbow on its IK circle, 75% hand-axis (forearm continues behind the wrist), bounded-rate
  rotation smoothing (no flips), capped at max(shoulder, wrist) + 5 cm
- PhysicsArms.SetHeld: held prop ignores that side's arm segments (PropGrabber calls it)
- ArmSpike: stretch settle, hug elbows out (0.17 m vs 0.02 m old pole), held-anvil stability;
  all six spikes PASS x3

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

scripts/BodyMath.cs.uid

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+uid://jcnflxkhghjy