MP: FIX phantom walk-in-place -- master sends a pose record on came-to-rest (task #50)

The linear pose send-gate watched only POSITION drift + the 2s heartbeat.
When a mech stops, its localOrigin is pinned and the master's own dead-reckon
projection re-bases to it each frame, so error->0 and NO pose record fires --
leaving the peer holding the last WALKING velocity (updateVelocity). The
replicant then dead-reckons + animates a phantom walk-in-place until the 2s
heartbeat (stretched to ~30s when the master window is OS-throttled to ~8fps
while backgrounded during solo two-window testing).

Add the symmetric linear 'came-to-rest' trigger -- the exact analog of the
angular (live yaw-rate==0 && replicated yaw-rate!=0) resync trigger already
present in the type-4 gate: fire a pose record the frame the live horizontal
speed collapses while the last-SENT speed was still non-zero, so the replicant
receives velocity=0 immediately instead of waiting for the heartbeat.

Diagnosis established via BT_WIRE tx0/rx0 probes: records deliver 1:1
(239 tx =~ 240 rx), velocities match -- NOT a replication stall; the master
simply never transmitted the stop. Also confirmed the dominant choppiness is
OS window-throttling (master 8fps backgrounded vs observer 63fps), a
two-windows-on-one-box test artifact absent on real pod hardware.

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

game/reconstructed/mech.cpp

@@ -1810,6 +1810,13 @@ void
 			updateVelocity.angularMotion = savedAngV;	// FUN_00408440
 			poseSyncLatch = 1;							// @0x77c -- arm the dead-reckon re-base
 
+			if (getenv("BT_WIRE"))
+				DEBUG_STREAM << "[rx0] ent=" << GetEntityID()
+					<< " uvLin=(" << updateVelocity.linearMotion.x << ","
+					<< updateVelocity.linearMotion.z << ")"
+					<< " uPos=(" << updateOrigin.linearPosition.x << ","
+					<< updateOrigin.linearPosition.z << ")\n" << std::flush;
+
 			if (bodyAnimationState == 2 || bodyAnimationState == 3)	// @0x728
 			{
 				projectedVelocity.linearMotion  = ZeroVector;	// FUN_0040a7f4(0x298, zeroMotion)