Windows port Phase 1: real cpal/WASAPI audio backend

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>

Cargo.toml

@@ -61,6 +61,6 @@ rfd = { version = "0.17", default-features = false, features = ["xdg-portal"] }
 [target.'cfg(windows)'.dependencies]
 # Native file picker using the built-in Win32 dialog backend on Windows.
 rfd = { version = "0.17", default-features = false }
-# NOTE: the Windows audio backend (cpal/WASAPI) lands in Phase 1. Until then the
-# Windows build uses the no-op `CpalBackend` stub in `src/audio/cpal_impl.rs`,
-# which needs no extra dependency.
+# Windows audio backend: cpal drives WASAPI for capture/playback behind the
+# AudioBackend trait (src/audio/cpal_impl.rs). The Linux counterpart is pipewire.
+cpal = "0.15"