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>
