W4 (WIP): dep-free resampler + capture/config wiring (playback pending)

- src/audio/resample.rs: pure linear PushResampler (capture) +
  StereoPullResampler (playback pull), 6 unit tests green on Linux.
- choose_config: prefer native 48kHz, else fall back to device default
  config and convert at the boundary instead of hard-erroring.
- run_capture: resample device-rate mono -> 48kHz on the drain thread.
- i16<->f32 helpers. Playback build_output remap still TODO (Codex).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/audio/mod.rs

@@ -61,6 +61,10 @@ pub mod gate;
 pub mod limiter;
 pub mod multitrack;
 pub mod pan;
+// Linear resamplers used by the Windows/cpal backend (W4). Platform-neutral and
+// pure, so it builds (and its tests run) everywhere even though only the cpal
+// backend wires it in.
+pub mod resample;
 #[cfg(target_os = "linux")]
 pub mod echo_cancel;
 #[cfg(target_os = "linux")]