feat(class/audio): rework TUH_AUDIO into a WASAPI/ALSA-like stream API

Provide a high-level audio streaming API over UAC 1.0 devices while
keeping the USB topology private: applications select supported
{format, sample_rate, channels} configurations per logical stream, and
the driver owns the mapping to AS interface, alternate setting, and
endpoint.

- One logical stream per direction per instance; multiple AS interfaces
  and alternate settings in a direction are merged into the stream's
  configuration list (discrete tuples; continuous ranges exposed as a
  single configuration at the top rate)
- Asynchronous tuh_audio_configure(): SET_INTERFACE to the selected
  alternate setting, open/reconfigure the endpoint, set the sampling
  frequency when supported, initialize the FIFO and packet scheduler,
  then invoke the completion callback
- Frame-based FIFO streaming: tuh_audio_read()/tuh_audio_write() queue
  whole frames; the driver owns transfer replenishment and fractional
  packet scheduling (44.1 kHz pays back the 0.1 frame/ms remainder via
  an accumulator for exact average pacing)
- tuh_audio_start()/tuh_audio_stop() activate/deactivate the stream
  interface through SET_INTERFACE (alt n / alt 0)

Driver correctness fixes:
- Parse only the AC header's interface collection; MIDI Streaming and
  other subclasses are skipped
- Keep every discrete format as a separate configuration; endpoints are
  opened only for the alternate setting selected by tuh_audio_configure()
- Check tuh_interface_set() return values and SET_INTERFACE transfer
  results instead of ignoring failures
- Validate instance state, direction, buffers, and frame counts in every
  transfer API
- Feature Unit requests use the control's real width (mute/AGC/loudness
  1 byte, others 2 bytes) and convert multibyte values to host order
- Failed/stalled/aborted isochronous transfers reach only the error
  callback, never the capture/playback callbacks

The audio_host example uses the new API: 48 kHz stereo by default,
automatic stream restart on error callbacks, a sine test tone on the
playback stream, and periodic mic-only / spk-only / echo phase switching.

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