MP: peer true-mirror cadence -- drive the body clip from actual replicated motion (task #50)

Toward a truer mirror (user: peer 'hesitates/skips on accel/decel, master has
smoother transitions').  The body channel FOLLOWS the master's replicated STATE
(96a896a) but its CADENCE (bodyCycleSpeed) is still slewed LOCALLY toward the
replicated commanded speed -- which drifts from the master's actual cadence, so
the animation phase wanders until a record snaps it (the hesitation).

FIX (replicant-gated, single-player untouched): peerMirrorSpeed member = the
actual replicated ground speed (|updateVelocity|, set in the peer branch); the
body channel walk (6/7) + reverse (0xc/0xd) cases override bodyCycleSpeed with
it (clamped to the state's band) right before the clip advance, so the peer's
clip advances at the master's REAL rate instead of a locally-drifting slew.
peerMirrorSpeed = -1 on the master/single-player -> the authentic slew runs.
BT_NO_MIRROR_CAD / BT_PEER_LEGCH revert.

Verified autonomous (through-zero sweep): no crash, body channel advances forward
(back-steps ~0), feet track (position ratio 1.05, maxStep 1.68u).  Perceived
smoothness is the visual test (harness saturates).

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

game/reconstructed/mech.cpp

@@ -916,6 +916,7 @@ Mech::Mech(
 	poseSyncLatch = 0;									// @0x77c (was absorbed Wword(0x1df))
 	heatLevelSnapshot = 0;								// @0x780 (type-7 deadband baseline)
 	mirrorBodyAdv  = 0.0f;								// master send-mirror gait travel
+	peerMirrorSpeed = -1.0f;							// peer true-mirror cadence (disabled by default)
 	angMirrorYaw   = 0.0f;								// scalar peer-yaw mirror (port addition)
 	angMirrorRate  = 0.0f;
 	angMirrorValid = 0;

game/reconstructed/mech.hpp

@@ -631,6 +631,12 @@ public:
 	// predicts (gait), instead of the constant-velocity deadReckoner. This is the
 	// authentic FUN_004a9b5c mirror = IntegrateMotion(mj=0) on projectedOrigin.
 	Scalar		mirrorBodyAdv;				// body cycleDistance this frame (master)
+	// Peer true-mirror cadence: the replicant sets this to the actual replicated
+	// ground speed (|updateVelocity|) each frame so the body clip advances at the
+	// master's real rate instead of the locally-slewed bodyCycleSpeed (which drifts
+	// and makes the animation phase wander -> the accel/decel hesitation).  -1 =
+	// disabled (master/single-player always -1, so the authentic slew runs).
+	Scalar		peerMirrorSpeed;
 	Scalar		angMirrorYaw;				// yaw at the last type-4 send (rad)
 	Scalar		angMirrorRate;				// yaw rate sent with it (rad/s)
 	Time		angMirrorTime;				// lastPerformance at that send

game/reconstructed/mech2.cpp

@@ -999,6 +999,9 @@ Scalar
 			if (bodyCycleSpeed > bodyTargetSpeed)		bodyCycleSpeed = bodyTargetSpeed;
 			if (bodyCycleSpeed > walkStrideLength)		bodyCycleSpeed = walkStrideLength;
 		}
+		if (peerMirrorSpeed >= 0.0f)			// peer: cadence == actual mirrored ground speed
+			bodyCycleSpeed = (peerMirrorSpeed < standSpeed) ? standSpeed
+				: ((peerMirrorSpeed > walkStrideLength) ? walkStrideLength : peerMirrorSpeed);
 		distance = bodyAnimation.Advance(
 			time_slice * (bodyCycleSpeed / walkStrideLength) * globalTimeScale, loop);
 		break;
@@ -1019,6 +1022,9 @@ Scalar
 			if (bodyCycleSpeed > bodyTargetSpeed)		bodyCycleSpeed = bodyTargetSpeed;
 			if (bodyCycleSpeed > reverseSpeedMax2)		bodyCycleSpeed = reverseSpeedMax2;
 		}
+		if (peerMirrorSpeed >= 0.0f)			// peer: cadence == actual mirrored ground speed
+			bodyCycleSpeed = (peerMirrorSpeed < reverseSpeedMax) ? reverseSpeedMax
+				: ((peerMirrorSpeed > reverseSpeedMax2) ? reverseSpeedMax2 : peerMirrorSpeed);
 		distance = bodyAnimation.Advance(
 			time_slice * (bodyCycleSpeed / reverseStrideLength) * globalTimeScale, loop);
 		break;
@@ -1159,6 +1165,9 @@ Scalar
 			if (bodyCycleSpeed > bodyTargetSpeed)		bodyCycleSpeed = bodyTargetSpeed;
 			if (bodyCycleSpeed > walkStrideLength)		bodyCycleSpeed = walkStrideLength;
 		}
+		if (peerMirrorSpeed >= 0.0f)			// peer: cadence == actual mirrored ground speed
+			bodyCycleSpeed = (peerMirrorSpeed < standSpeed) ? standSpeed
+				: ((peerMirrorSpeed > walkStrideLength) ? walkStrideLength : peerMirrorSpeed);
 		distance = bodyAnimation.Advance(
 			time_slice * (bodyCycleSpeed / walkStrideLength) * globalTimeScale, loop);
 		break;
@@ -1179,6 +1188,9 @@ Scalar
 			if (bodyCycleSpeed > bodyTargetSpeed)		bodyCycleSpeed = bodyTargetSpeed;
 			if (bodyCycleSpeed > reverseSpeedMax2)		bodyCycleSpeed = reverseSpeedMax2;
 		}
+		if (peerMirrorSpeed >= 0.0f)			// peer: cadence == actual mirrored ground speed
+			bodyCycleSpeed = (peerMirrorSpeed < reverseSpeedMax) ? reverseSpeedMax
+				: ((peerMirrorSpeed > reverseSpeedMax2) ? reverseSpeedMax2 : peerMirrorSpeed);
 		distance = bodyAnimation.Advance(
 			time_slice * (bodyCycleSpeed / reverseStrideLength) * globalTimeScale, loop);
 		break;

game/reconstructed/mech4.cpp

@@ -2148,6 +2148,17 @@ void
 				// master's exact clips.  Swapping the peer to the body channel RESTORES the
 				// original single-channel design (no new netcode).  BT_PEER_LEGCH=1 = old.
 				static const int s_peerLegCh = getenv("BT_PEER_LEGCH") ? 1 : 0;
+				// TRUE-MIRROR CADENCE: drive the body clip from the ACTUAL replicated ground
+				// speed so the animation phase tracks the master exactly instead of drifting
+				// via the local bodyCycleSpeed slew (the accel/decel hesitation).  Consumed in
+				// AdvanceBodyAnimation walk/reverse.  -1 disables -> authentic slew.
+				if (!s_peerLegCh && !getenv("BT_NO_MIRROR_CAD"))
+				{
+					const Vector3D &mcv = updateVelocity.linearMotion;
+					peerMirrorSpeed = (Scalar)sqrtf((float)(mcv.x * mcv.x + mcv.z * mcv.z));
+				}
+				else
+					peerMirrorSpeed = -1.0f;
 				// TURN-STEP ARMING for the body channel: the body Standing case only enters
 				// walk/reverse/stand -- it never arms the turn-in-place clip (state 4); on the
 				// master that arming comes from the LEG Standing case cross-arming both