Add AudioEngine::playSound2DStoppable() + stopSound() so callers can
hold a handle and cancel playback early. SpellSoundManager::playPrecast()
now stores the handle in activePrecastId_; stopPrecast() cuts the sound.
playCast() calls stopPrecast() before playing the release sound, so the
channeling audio never bleeds past cast time. SMSG_SPELL_FAILURE and
SMSG_CAST_FAILED both call stopPrecast() so interrupted casts silence
immediately.
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)
