Introduces AudioCommand enum (Play, Stop, SetVolume, StopAll, Shutdown) and AudioSystem that spawns a dedicated audio thread. On Windows the thread drives WasapiDevice with a 5ms mix-and-write loop; on other platforms it runs in silent null mode. lib.rs exports wasapi (windows) and audio_system modules with AudioSystem re-exported at crate root. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>