Replace the Phase 0 no-op CpalBackend stub with a working cpal backend
(WASAPI on Windows), preserving the exact PipeWire AudioBackend contract
so the mixer/encoder/jitter pipeline is unchanged.
- Capture: input stream -> downmix to mono -> 960-sample (20ms) i16 frames
-> tx, matching the encoder/jitter frame size.
- Playback: 200ms stereo ring prefilled to PLAYBACK_TARGET_SAMPLES; the
output callback drains it (silence on underrun) while the owning thread
feeds it from rx. ring_fill is the exact delta-maintained occupancy
counter (fetch_add on push, fetch_sub on pop), preserving the clock-paced
production design (not ringbuf's stale occupied_len).
- cpal::Stream is !Send, but AudioBackend is Send+Sync and shared via Arc,
so each stream lives on its own owning thread (built/played/dropped
there); the struct holds only the running flag + JoinHandle. stop()
flips the flag and joins.
- Generic over F32/I16/U16 sample formats; device selected by name else
default; requires a native 48kHz config (clear error otherwise, no
resampling yet). Mirrors the PipeWire drain_loop and playout-health line.
- Cargo.toml: add cpal 0.15 under cfg(windows).
Verified by temporarily compiling cpal_impl against real cpal on Linux/ALSA:
build + clippy clean, 6/6 cpal_impl unit tests pass. Reverted to windows-only
gating; shipped Linux state green (316/316). Runtime/WASAPI end-to-end is
unverified and pending a Windows host (plan M2).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
molluskandClaude Opus 4.8