Add mount idle fidget animations and ambient sounds

Implements WoW-style mount idle behavior when player is stationary:
- Fidget animations: discovered via property search (non-looping, 500-1500ms, stationary, IDs 1-10)
- Triggers random fidget every 6-12 seconds when standing still
- Ambient idle sounds: snorts/breaths for ground mounts, soft wing sounds for flyers
- Triggers random idle sound every 8-15 seconds when stationary
- Both systems reset timers on movement to avoid triggering while riding

include/audio/mount_sound_manager.hpp

@@ -54,6 +54,7 @@ public:
     void playRearUpSound();   // Rear-up flourish (whinny/roar)
     void playJumpSound();     // Jump start (grunt/snort)
     void playLandSound();     // Landing (thud/hoof)
+    void playIdleSound();     // Ambient idle (snort/stomp/breath)
 
     bool isMounted() const { return mounted_; }
     void setVolumeScale(float scale) { volumeScale_ = scale; }