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)

game/reconstructed/mech4.cpp

@@ -3414,12 +3414,38 @@ void
 			{
 				Scalar posDb = (updatePositionDeadband > 0.0f)
 					? updatePositionDeadband : 0.04f;
+				// LINEAR "came-to-rest" trigger -- the symmetric analog of the
+				// angular (live yaw-rate==0 && replicated yaw-rate!=0) resync
+				// trigger in the type-4 gate below.  The position deadband alone
+				// does NOT catch a stop: once the master halts, its localOrigin is
+				// pinned and its OWN dead-reckon projection is re-based to it each
+				// frame, so error->0 and no pose record fires -- leaving the peer
+				// holding the last WALKING velocity (updateVelocity), which the
+				// replicant keeps dead-reckoning + animating as a phantom walk
+				// until the 2s heartbeat.  Fire a pose record the frame the live
+				// horizontal speed collapses while the last-SENT speed was still
+				// non-zero, so the replicant gets velocity=0 immediately.
+				const Scalar liveSpdSq =
+					(Scalar)(localVelocity.linearMotion.x * localVelocity.linearMotion.x
+					+ localVelocity.linearMotion.z * localVelocity.linearMotion.z);
+				const Scalar sentSpdSq =
+					(Scalar)(updateVelocity.linearMotion.x * updateVelocity.linearMotion.x
+					+ updateVelocity.linearMotion.z * updateVelocity.linearMotion.z);
+				const Logical cameToRest = (liveSpdSq < 0.01f && sentSpdSq > 0.01f);
 				if (
 					error.LengthSquared() > posDb
 					|| lastPerformance - lastUpdate > 2.0f
+					|| cameToRest
 				)
 				{
 					ForceUpdate();					// type 0: pose record
+					if (getenv("BT_WIRE"))
+						DEBUG_STREAM << "[tx0] errSq=" << error.LengthSquared()
+							<< " posDb=" << posDb
+							<< " hb=" << (float)(lastPerformance - lastUpdate)
+							<< (cameToRest ? " REST" : "")
+							<< " lvLin=(" << localVelocity.linearMotion.x << ","
+							<< localVelocity.linearMotion.z << ")\n" << std::flush;
 				}
 			}
 			// ORIENTATION rides ONLY the type-4 resync record (the authentic