Audio: restore the dropped ExecuteWatchers poll -- polled audio watchers were ALL dead (task #50)

User: "when I start moving all the sounds fade away, no footsteps."  Diagnosis
chain (all empirical, BT_AUDIO_SPATIAL/BT_ATTRBIND_LOG traces):
- The audio head DOES track the mech (listener-relative positioning verified
  while driving) -- not a spatial bug.
- The idle sounds correctly STOP on leaving the standing state; the MOVING
  sounds never started because every POLLED audio watcher was dead:
  the reconstructed Mech::PerformAndWatch replaced the engine performance but
  dropped the ExecuteWatchers() step from the engine tail (Simulation::
  PerformAndWatch = Perform -> ExecuteWatchers -> WriteSimulationUpdate).
  Only PUSHED StateIndicator watchers (SetState->Execute) ever fired -- which
  is exactly why state sounds worked but footsteps/motion-scaled audio didn't.
  FIX: poll ExecuteWatchers() (AreWatchersDelayed-gated) before the update
  write.  Immediately unlocks the polled family: MissileLoaded01/LaserLoaded01
  ready dings, ProgramButton01, motion scales (PlayNote 15 -> 30 per run).

- FootStep (0x1e) backed REAL: was the inert attrPad (an AudioLogicalTrigger
  polls it -- threshold-crossing pulse per foot plant).  New footStep member,
  pulsed by SetBodyAnimation on entering any locomotion clip 1..0x17 (runtime
  stride alternation MEASURED as body states 12<->13 -- the enum-name numbering
  does not match runtime clip ids), decayed by IntegrateMotion (~1/3 s).
- Footstep volume is speed-scaled (AudioMotionScale on LocalVelocity): at
  standstill the transient start computes volume 0 and the renderer drops it
  (LowAudioVolumeThreshold) -- so footfalls are audible at real walking speed.

Diagnostics kept (BT_AUDIO_SPATIAL): spatial dist/vol per source, CLIPPED/DROP/
START at the request gate, vol=0 factor breakdown, scale->0 sends, trigger
notifications, watcher poll-rate probe.

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

engine/MUNGA/AUDSRC.cpp

@@ -1,3 +1,4 @@
+#include <cstdlib>
 #include "munga.h"
 #pragma hdrstop
 
@@ -416,6 +417,12 @@ AudioControlValue
 		volume_scale;
 		
 	volume_scale = volumeScale * volumeMixScale * volumeCompressionScale;
+	if (getenv("BT_AUDIO_SPATIAL") && volume_scale <= 0.0f) {
+		static int s_z=0; if (s_z++<40)
+			DEBUG_STREAM << "[spatial] vol=0 breakdown src=" << (void*)this
+				<< " volumeScale=" << volumeScale
+				<< " mixScale=" << volumeMixScale
+				<< " compression=" << volumeCompressionScale << "\n" << std::flush; }
 	Clamp(volume_scale, MinAudioVolume, MaxAudioVolume);
 	return volume_scale;
 }