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.

src/rendering/renderer.cpp

@@ -730,9 +730,6 @@ bool Renderer::initialize(core::Window* win) {
     spellSoundManager = std::make_unique<audio::SpellSoundManager>();
     movementSoundManager = std::make_unique<audio::MovementSoundManager>();
 
-    // TODO Phase 6: Vulkan underwater overlay, post-process, and shadow map
-    // GL versions stubbed during migration
-
     // Create secondary command buffer resources for multithreaded rendering
     if (!createSecondaryCommandResources()) {
         LOG_WARNING("Failed to create secondary command buffers — falling back to single-threaded rendering");