Implement MPQ-path sound loading in AudioEngine

Add AssetManager hookup to AudioEngine so the path-based playSound2D/3D
overloads can load files on demand rather than requiring preloading.

- Add setAssetManager() to AudioEngine (called during world load alongside
  other audio manager initializations)
- playSound2D(mpqPath) now calls assetManager->readFile() then delegates
  to the vector<uint8_t> overload (removes the "not yet implemented" warning)
- playSound3D(mpqPath, position) same — delegates to the fully spatialized
  vector overload (was previously silently falling back to 2D)

include/audio/audio_engine.hpp

@@ -11,6 +11,7 @@ struct ma_engine;
 struct ma_sound;
 
 namespace wowee {
+namespace pipeline { class AssetManager; }
 namespace audio {
 
 /**
@@ -32,6 +33,9 @@ public:
     void setMasterVolume(float volume);
     float getMasterVolume() const { return masterVolume_; }
 
+    // Asset manager (enables sound loading by MPQ path)
+    void setAssetManager(pipeline::AssetManager* am) { assetManager_ = am; }
+
     // 3D listener position (for positional audio)
     void setListenerPosition(const glm::vec3& position);
     void setListenerOrientation(const glm::vec3& forward, const glm::vec3& up);
@@ -81,6 +85,8 @@ private:
     glm::vec3 listenerForward_{0.0f, 0.0f, -1.0f};
     glm::vec3 listenerUp_{0.0f, 1.0f, 0.0f};
 
+    pipeline::AssetManager* assetManager_ = nullptr;
+
     // miniaudio engine (opaque pointer)
     ma_engine* engine_ = nullptr;
 };