Chasing the ~2.9u residual snap: instrumented BT_MIRDIV and isolated it to the
PEER side (master send-mirror error is accurate, mean 0.39u < 0.55u threshold;
peer drifted 3-11u from received authority). Root: the master models the peer
with the BODY channel but the peer moves with the LEG channel -- slightly
different travel/frame -- so the master deadband does not fire exactly when the
peer drifts, and the fixed ~1/3s offset decay bled slower than accel/decel drift
accumulated. A hard ground-snap (authentic FUN_004ab1c8:14985) popped instead
(records not perfectly dense on the one-box relay).
FIX: error-proportional absorption -- bleed the offset to updateOrigin at a rate
that scales with the error (k 0.15..0.6/frame): small steady offset absorbed
gently (no foot-pop), large speed-change drift caught in a few frames.
Measured (through-zero sweep): peer drift 11u->2.1u, maxStep 2.9u->1.77u, ratio
1.043->1.036. Remaining <=1.8u snaps are the leg-vs-body channel-shape mismatch;
fully closing needs a leg-channel send-mirror (second instance) -- deferred.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
