Audio System + Drivers (#562)

Drivers/audio-stream-module/README.md

@@ -0,0 +1,22 @@
+# Audio stream module
+
+Defines `AUDIO_STREAM_TYPE` and `AudioStreamApi`: the high-level, full-duplex
+audio API that apps (including ELF side-loaded apps) and `AudioService` should
+use instead of talking to an `AUDIO_CODEC_TYPE` device directly. Binds to one
+input-capable and/or one output-capable codec device (found automatically at
+start; see `Drivers/audio-codec-module`) and adds on top of it:
+
+- Resampling, so the same app code works at any requested sample rate
+  regardless of what rate the bound codec natively runs at.
+- Shared volume/mute/enable state per direction, with a change callback
+  (`AudioStreamChangeCallback`) that `AudioService` subscribes to.
+- A single always-present device (`audio-stream0`), constructed unconditionally
+  at module-start time before any devicetree codec exists, and bound lazily on
+  first use -- so boards with no audio hardware at all still get a harmless,
+  inert device rather than nothing.
+
+`open_input`/`open_output` return a stream handle; `read`/`write` on that
+handle are blocking and must be called from the caller's own task, never from
+the main/LVGL thread.
+
+License: [Apache v2.0](LICENSE-Apache-2.0.md)