Add dedicated snort and whinny idle sounds with longer intervals

Created specific idle sound pool using only horse snorts and whinnies.
Re-enabled idle sounds with much longer interval (20-40 seconds).

Changes:
- Added horseIdleSounds_ pool: mHorseStand3A (snort) + mHorseAggroA (whinny)
- Updated playIdleSound() to use dedicated pool instead of mixed breath sounds
- Increased idle sound interval from 8-15s to 20-40s (less frequent)
- Removed flying mount idle sounds (too aggressive)
- Increased volume slightly (0.35x) for better audibility

include/audio/mount_sound_manager.hpp

@@ -84,6 +84,7 @@ private:
     std::vector<MountSample> horseMoveSounds_;
     std::vector<MountSample> horseJumpSounds_;   // Jump effort sounds
     std::vector<MountSample> horseLandSounds_;   // Landing thud sounds
+    std::vector<MountSample> horseIdleSounds_;   // Snorts and whinnies for idle
 
     // Sound state tracking
     bool playingMovementSound_ = false;