MP: COUPLED peer motion -- authentic gait-driven position + master gait send-mirror (task #50, env-gated)

Finishes the coupled IntegrateMotion path the decomp workflow (wh1h5gnmc, 3
make-or-break claims CONFIRMED by adversarial verify) proved authentic:
 - Peer LINEAR position is SINGLE-SOURCE gait: IntegrateMotion integrates the
   body-channel cycleDistance into projectedOrigin.linear@0x260, copied verbatim
   to localOrigin -- NOT velocity dead-reckon (that is angular/heading ONLY).
   0x260 'motionDelta' and 'projectedOrigin' are literally the same field; the
   'contradiction' was our reconstruction's two misnamed shadows.
 - Master and peer run the SAME predictor; the master's SEND-mirror must run it
   too (binary FUN_004a9b5c @0x4aab9c) so a gait-driven peer stays anchored. Our
   mirror used the constant-velocity deadReckoner + overwrote bodyTargetSpeed
   live every frame -> could not model a gait peer (the tug-of-war).
 - T4 CONFIRMED: both channels pose the full skeleton (same JointedMover); keep
   the peer LEG channel (our body channel is unbound on the peer).

CHANGES (all behind BT_MASTER_GAITMIRROR / BT_ROOT_POS, DEFAULT OFF -- zero
change to shipped behavior until visually confirmed):
 - mech4.cpp: send-mirror advances projectedOrigin by the mj=0 body channel
   travel (mirrorBodyAdv) rotated by heading + last-sent angular vel, re-seeded
   to localOrigin on each send, instead of the deadReckoner; and stops the
   per-frame live bodyTargetSpeed overwrite so the mirror slews toward last-sent.
 - mech.hpp/mech.cpp: mirrorBodyAdv member.

Measured A/B (autonomous through-zero sweep circle, the speed-change regime):
   velocity two-source (current default): ratio 1.0022 BUT user-visible glitch
   coupled, NO mirror (dense):            ratio 1.86  (tug-of-war)
   coupled + gait mirror, dense OFF:      ratio 1.14, back-steps 0.1%
   coupled + gait mirror, dense ON:       ratio 1.043, back-steps 0.1%, maxStep 2.9u
The single-source coupling is proven (backward-stepping 0.1% vs the split's
churn). Residual 2.9u occasional snap = leg(peer)-vs-body(mirror) channel
mismatch, second-order. Awaiting visual confirmation before default promotion.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

game/reconstructed/mech.cpp

@@ -915,6 +915,7 @@ Mech::Mech(
 	Wword(0xe6)  = 0;
 	poseSyncLatch = 0;									// @0x77c (was absorbed Wword(0x1df))
 	heatLevelSnapshot = 0;								// @0x780 (type-7 deadband baseline)
+	mirrorBodyAdv  = 0.0f;								// master send-mirror gait travel
 	angMirrorYaw   = 0.0f;								// scalar peer-yaw mirror (port addition)
 	angMirrorRate  = 0.0f;
 	angMirrorValid = 0;