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)