Implement bird/cricket ambient sounds and remove stale renderer TODO

- Add birdSounds_ and cricketSounds_ AmbientSample vectors to
  AmbientSoundManager, loaded from WoW MPQ paths:
  BirdAmbience/BirdChirp01-06.wav (up to 6 variants, daytime) and
  Insect/InsectMorning.wav + InsectNight.wav (nighttime). Missing files
  are silently skipped so the game runs without an MPQ too.
- updatePeriodicSounds() now plays a randomly chosen loaded variant
  at the scheduled interval instead of the previous no-op placeholder.
- Remove stale "TODO Phase 6: Vulkan underwater overlay" comment from
  Renderer::initialize(); the feature has been fully implemented in
  renderOverlay() / the swim effects pipeline since that comment was
  written.

include/audio/ambient_sound_manager.hpp

@@ -114,6 +114,8 @@ private:
     std::vector<AmbientSample> windSounds_;
     std::vector<AmbientSample> tavernSounds_;
     std::vector<AmbientSample> blacksmithSounds_;
+    std::vector<AmbientSample> birdSounds_;
+    std::vector<AmbientSample> cricketSounds_;
 
     // Weather sound libraries
     std::vector<AmbientSample> rainLightSounds_;