MP: replicant gait animation -- peer mechs walk instead of sliding (task #50)

A peer's replicant mech slid around as a frozen statue: DeadReckon moved
localOrigin but nothing animated the legs.  Reconstruct the replicated-motion
display loop: after DeadReckon, derive a SIGNED speed demand from the
replicated velocity (the master publishes worldLinearVelocity in every update
record, Mover::WriteUpdateRecord MOVER.cpp:759; the replicant stores it in
updateVelocity.linearMotion :712; sign = forward dot against the -Z facing),
write it to controlsMapper->speedDemand (the LEG channel's live demand
source), and advance the LEG state machine for its JOINT writes only --
travel stays with DeadReckon, so position always follows the master and the
clip cadence matches the replicated speed.

Two replicant-only initialization holes found live (the master's equivalents
are primed per-frame in the PLAYER-only drive block):
- globalTimeScale/idleStrideScale read 0 -> clip advance time dt*0, the leg
  machine engaged (state 11) but the clip froze (legCycle stuck 0);
- reverseSpeedMax2 (the run-cycle rise-clamp @0x7a0; LoadLocomotionClips does
  NOT set it) read debug-heap garbage -> legCycleSpeed clamped to -4.3e8 on
  entering run state 13.  Primed both; legCycleSpeed sane-banded once.

VERIFIED 2-node (BT_GOTO driver + BT_REPL_LOG observer): the replicant runs
the full gait lifecycle -- stand -> walk(5,7) -> run(10,12), legCycle 7.5->34
tracking the master's speed, wind-down(8), Standing(0) when the master stops.
Solo un-regressed (goto->aim->kill chain intact, no crash).

projectedVelocity source-swap (the binary publishes the BODY channel's
smoothed projection instead of the actual travel velocity) evaluated and
DEFERRED: it risks a verified working replication feed for a smoothing
nuance; recorded in multiplayer.md.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

game/reconstructed/mech4.cpp

@@ -1194,6 +1194,51 @@ void
 		{
 			DeadReckon(dt);							// engine: reckoner + lerp
 			localToWorld = localOrigin;
+
+			// REPLICANT GAIT (task #50): animate the peer mech's legs at the
+			// REPLICATED speed -- without this the replicant slides around as a
+			// frozen statue.  The master publishes worldLinearVelocity in every
+			// update record (Mover::WriteUpdateRecord, MOVER.cpp:759); the
+			// replicant stores it in updateVelocity.linearMotion (:712).  Derive
+			// the signed speed demand from it (forward = local -Z; a negative
+			// forward dot = the master is reversing), feed the LEG channel's live
+			// demand source, and advance the leg state machine for its JOINT
+			// writes only -- TRAVEL stays with DeadReckon (the returned distance
+			// is discarded), so position always follows the master and the clip
+			// cadence matches the replicated speed (residual foot-slip is the
+			// inherent dead-reckoning artifact).  The state machine self-arms
+			// stand->walk / winds down from the demand exactly as on the master.
+			if (!IsMechDestroyed() && controlsMapper != 0)
+			{
+				const Vector3D &wv = updateVelocity.linearMotion;
+				float spd = sqrtf((float)(wv.x * wv.x + wv.z * wv.z));
+				UnitVector zAxR;
+				localToWorld.GetFromAxis(Z_Axis, &zAxR);
+				const float fdot = -((float)wv.x * (float)zAxR.x
+								   + (float)wv.z * (float)zAxR.z);	// mech faces -Z
+				controlsMapper->speedDemand = (fdot < 0.0f) ? -spd : spd;
+				// Prime the same clip-advance scalars the master's gait block sets
+				// each frame -- uninitialized on a replicant they read 0, freezing
+				// the clip at advance-time dt*0 (observed: legState engaged at 11,
+				// legCycle stuck 0).  Same forwardCycleRate floor as the master
+				// (the model-record decode reads ~1 u/s^2 -- a 55s ramp; floor 25).
+				globalTimeScale = 1.0f;
+				idleStrideScale = 1.0f;
+				if (forwardCycleRate < 25.0f) forwardCycleRate = 25.0f;
+				// reverseSpeedMax2@0x7a0: the run-cycle rise-CLAMP (leg case 12/13);
+				// LoadLocomotionClips doesn't set it, so a replicant reads 0xCDCD
+				// (-4.3e8) and the clamp CLOBBERS legCycleSpeed the moment the run
+				// cycle engages (observed live: legCycle=-4.31602e+08, state 13
+				// stuck).  Same heal as the master's drive block.
+				reverseSpeedMax2 = reverseStrideLength;
+				// legCycleSpeed itself is also debug-heap garbage until a walk
+				// state first writes it (Standing never touches it); sane-band it
+				// once so a direct run-state entry can't slew from -4.3e8.
+				if (legCycleSpeed < -100.0f || legCycleSpeed > 200.0f)
+					legCycleSpeed = 0.0f;
+				(void)AdvanceLegAnimation(dt);		// joints only; travel = DeadReckon
+			}
+
 			if (getenv("BT_REPL_LOG"))
 			{
 				static float s_replLog = 0.0f; s_replLog += dt;
@@ -1203,7 +1248,11 @@ void
 					DEBUG_STREAM << "[repl] mech " << (long)GetEntityID()
 						<< " pos=(" << localOrigin.linearPosition.x << ", "
 						<< localOrigin.linearPosition.y << ", "
-						<< localOrigin.linearPosition.z << ")\n" << std::flush;
+						<< localOrigin.linearPosition.z << ")"
+						<< " vel=(" << updateVelocity.linearMotion.x << ","
+						<< updateVelocity.linearMotion.z << ")"
+						<< " legState=" << (int)legStateAlarm.GetLevel()
+						<< " legCycle=" << legCycleSpeed << "\n" << std::flush;
 				}
 			}
 		}