Add fountain positional ambient sounds

Implemented fountain sounds as positional audio emitters:

Technical details:
- Added fountainSounds_ library (FountainSmallMediumLoop.wav)
- Fountain type already existed in AmbientType enum but was unused
- 6 second loop interval for fountain sounds
- Volume at 0.8x water volume for gentle bubbling effect
- Max distance: 35 units (same as other water sources)
- Counted in activeWaterCount limit (max 3 water sources at once)
- Spatial 3D audio based on fountain position
- Can be placed via addEmitter(position, AmbientType::FOUNTAIN)

include/audio/ambient_sound_manager.hpp

@@ -106,6 +106,7 @@ private:
     std::vector<AmbientSample> waterSounds_;
     std::vector<AmbientSample> riverSounds_;
     std::vector<AmbientSample> waterfallSounds_;
+    std::vector<AmbientSample> fountainSounds_;
     std::vector<AmbientSample> windSounds_;
     std::vector<AmbientSample> tavernSounds_;
     std::vector<AmbientSample> blacksmithSounds_;