Module audio::audio_backend

audio: the interface between audio.sc and the per-API backends (audio_<api>[_<os>].sc). std-private.

Each backend is one AudioBackendVt_ (a table of functions) audio.sc picks by API id. A backend fills an AudioHost_'s device list in probe(), and opens/starts/stops an AudioStreamState_ that audio.sc allocated and filled with the request. On its real-time path it only calls audio_stream_process_() (convert + callback + convert, allocation- and lock-free) and the atomic flag helpers below.