Gait: FIX replicant trn regression -- gate master-perf turn-stop exit to masters

Today's authentic trn dispatcher folded the master-perf turn-STOP exit into leg/body
case 4.  But the master perf (FUN_004a9b5c) runs on MasterInstance mechs, NOT
replicants -- and a replicant derives turnDemand from the noisy REPLICATED yaw rate
(mech4.cpp:1968), which dips into the +-0.05 deadband between dead-reckon updates.  So
the exit kicked the peer out of trn every few frames -> the peer 'rotated as a statue'
+ jerky (user-reported regression from yesterday).  Gate both turn-stop exits to
GetInstance() != ReplicantInstance; the replicant advances the trn clip from its
replicated turn signal exactly as before the exit was added.  Also BT_REPL_TRN probe
(mech4.cpp) logs the replicant yawRate/turnDemand/legState.

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

game/reconstructed/mech2.cpp

@@ -693,8 +693,17 @@ Scalar
 			distance = 0.0f;
 			break;
 		}
-		if (mppr == 0
+		// MASTER ONLY (regression fix): the master-perf turn-stop exit is master-perf
-			|| (mppr->turnDemand <= 0.05f && mppr->turnDemand >= -0.05f))
+		// logic (FUN_004a9b5c runs on MasterInstance mechs, NOT replicants).  A replicant
+		// derives turnDemand from the REPLICATED yaw rate (mech4.cpp:1968) -- a noisy
+		// proxy that dips into the deadband between dead-reckon updates; running this
+		// exit on it kicked the peer out of trn every few frames -> the peer "rotated as
+		// a statue" + jerky (user-reported).  Gate to masters; a replicant leaves trn via
+		// the speed exits + its own turnDemand-driven re-entry, exactly as it did before
+		// this exit was added.
+		if (GetInstance() != ReplicantInstance
+			&& (mppr == 0
+				|| (mppr->turnDemand <= 0.05f && mppr->turnDemand >= -0.05f)))
 		{
 			legStateAlarm.SetLevel(0);					// turn stopped -> Standing (master-perf exit)
 			ForceUpdate(8);
@@ -932,8 +941,9 @@ Scalar
 				distance = 0.0f;
 				break;
 			}
-			if (bm == 0
+			if (GetInstance() != ReplicantInstance		// MASTER only (see leg twin)
-				|| (bm->turnDemand <= 0.05f && bm->turnDemand >= -0.05f))
+				&& (bm == 0
+					|| (bm->turnDemand <= 0.05f && bm->turnDemand >= -0.05f)))
 			{
 				bodyStateAlarm.SetLevel(0);				// turn stopped (leg-symmetric)
 				ForceUpdate(8);

game/reconstructed/mech4.cpp

@@ -1988,6 +1988,19 @@ void
 					legCycleSpeed = 0.0f;
 				(void)AdvanceLegAnimation(dt);		// joints only; travel = DeadReckon
 
+				if (getenv("BT_REPL_TRN"))
+				{
+					static float s_rt = 0.0f; s_rt += dt;
+					if (s_rt >= 0.15f)
+					{
+						s_rt = 0.0f;
+						DEBUG_STREAM << "[repltrn] yawRate=" << yawRate
+							<< " turnDemand=" << replMppr->turnDemand
+							<< " legState=" << (int)legStateAlarm.GetLevel()
+							<< " spd=" << replMppr->speedDemand << "\n" << std::flush;
+					}
+				}
+
 				// REPLICANT BEAMS (task #51): the emitters carry live replicated
 				// discharge state (Emitter::ReadUpdateRecord); draw them with the
 				// same per-weapon walk the master uses.