Add PlayMusicCallback to GameHandler so SMSG_PLAY_MUSIC (and the
vanilla 0x0103 alias) dispatch a soundId to the registered handler
instead of being silently consumed. Application.cpp registers the
callback, loads SoundEntries.dbc, resolves the first non-empty
Name+DirectoryBase into an MPQ path, and passes it to MusicManager
for non-looping playback. Resolves the TODO in the SMSG_PLAY_MUSIC
handler.
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