Audio: kill the per-footfall static -- smooth the PUBLISHED velocity (task #50)

User pinpointed it: the per-stride 'static' was NOT the footstep sample -- it
was the ambient engine hum being interrupted in time with the footfalls.
Diagnosis (live tape): EnginePower01 was caught in a rapid Play->Stop cycle
(80x/session).  The idle-hum is gated by authored LocalVelocity triggers (hum
ON in speed [1,20], OFF above 20) -- and the port's localVelocity is a RAW
per-frame position derivative that pulses with every stride (gait acceleration
+ ground-snap bob).  At walking speed |v| straddles the 20 band edge, so the
trigger Start/Stop-flapped the hum on every foot plant; at run |v| >> 20 keeps
it off (matching 'goes away at higher speeds').

Fix: ~0.25s exponential filter on the PUBLISHED localVelocity.linearMotion
(fwd + vertical) -- kills the stride ripple, tracks real speed changes within
a couple strides.  The physics worldLinearVelocity (StaticBounce, collision
damage pricing) is untouched; update records + the impact gate get the
smoother value (both benefit).

VERIFIED: EnginePower 1 play / 0 stops for the whole run (was 80 stop-cycles);
FootFall cycles normally per stride; gear shifts/windup lifecycle intact.

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

game/reconstructed/mech.cpp

@@ -925,6 +925,8 @@ Mech::Mech(
 	collisionTemporaryState = 0;
 	footStep                = 0;					// FootStep pulse (audio trigger)
 	footStepDecay           = 0;
+	fwdSpeedFiltered        = 0.0f;					// smoothed published velocity (see mech4 derive)
+	vertSpeedFiltered       = 0.0f;
 	{ extern void *g_btFootStepAddr; g_btFootStepAddr = &footStep; }	// DIAG: watcher-poll tracer target
 	radarRange          = 1000.0f;						// radar display scale (SetTargetRange is stubbed;
 														// 1km default zoom so contacts within ~500m show on

game/reconstructed/mech.hpp

@@ -848,6 +848,10 @@ protected:
 		// per-frame motion tick decays it back to 0 a few frames later, giving one
 		// clean rising edge per step.  [T2]
 		int				footStep;			// the Logical the matcher polls (1 = contact)
+		Scalar			fwdSpeedFiltered;	// smoothed |v| published via localVelocity (the raw
+		Scalar			vertSpeedFiltered;	// per-frame position derivative pulses per stride and
+											// flapped the audio's speed triggers -- engine-hum
+											// restarts on every footfall)
 		int				footStepDecay;		// MILLISECONDS until the pulse clears (time-based:
 											// the watcher polls ~10Hz and strides re-pulse ~2Hz,
 											// so the pulse must be one poll-period wide but end

game/reconstructed/mech4.cpp

@@ -3639,7 +3639,20 @@ void
 			Scalar fwdSpeed = (Scalar)sqrtf(
 				worldLinearVelocity.x * worldLinearVelocity.x +
 				worldLinearVelocity.z * worldLinearVelocity.z);
-			localVelocity.linearMotion  = Vector3D(0.0f, worldLinearVelocity.y, -fwdSpeed);
+			// SMOOTH the PUBLISHED velocity (localVelocity feeds the audio speed
+			// triggers, the update records, and the impact gate -- NOT the physics
+			// worldLinearVelocity above).  The raw per-frame derivative pulses per
+			// stride (gait acceleration + ground-snap bob): at walking speed the
+			// oscillation straddled the authored idle-hum band edge (speed 20) and
+			// Start/Stop-flapped the engine ambience on every footfall (heard as a
+			// static tick per step).  ~0.25s exponential filter kills the stride
+			// ripple while tracking real speed changes within a couple of strides.
+			{
+				Scalar alpha = dt / (dt + 0.25f);
+				fwdSpeedFiltered  += alpha * (fwdSpeed - fwdSpeedFiltered);
+				vertSpeedFiltered += alpha * (worldLinearVelocity.y - vertSpeedFiltered);
+			}
+			localVelocity.linearMotion  = Vector3D(0.0f, vertSpeedFiltered, -fwdSpeedFiltered);
 			localVelocity.angularMotion = Vector3D(0.0f, turn * authTurnRate, 0.0f);
 
 			// MASTER twin of [replmov]: the velocity this mech PUBLISHES vs its own