Windows port Phase 2: cpal device enumeration

Give the Windows device pickers a real device list (Phase 0/1 left pw_cli
returning nothing off-Linux) and generalize enumeration into a
platform-neutral interface.

- audio/mod.rs: move AudioDevice here (neutral home), gate pw_cli to
  cfg(unix), and re-export enumerate_audio_devices per-platform (pw_cli on
  unix, cpal_impl on windows). Also drop a now-stale "no-op stub" doc note.
- cpal_impl.rs: add enumerate_audio_devices() — iterate the cpal host's
  input + output devices into AudioDevice (name == description == the cpal
  friendly name, which is what resolve() matches target_node against, so a
  saved selection round-trips), sorted by description.
- pw_cli.rs: use super::AudioDevice instead of a local copy; parsing +
  tests unchanged.
- app/mod.rs: one-line import change; the device-picker logic is untouched.

Verified: shipped Linux state green (build --locked, clippy, 316/316,
pw_cli parse tests 6/6); the cpal enumerator compiles against real cpal via
the Linux/ALSA toggle. Runtime device listing on Windows is pending a real
host (M2/M3). WASAPI names are less stable than PipeWire node names, so a
saved device may not always round-trip (falls back to default).

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

src/audio/pw_cli.rs

@@ -1,18 +1,6 @@
+use super::AudioDevice;
 use std::process::Command;
 
-#[derive(Debug, Clone, PartialEq, Eq)]
-pub struct AudioDevice {
-    pub name: String,
-    pub description: String,
-    pub is_input: bool,
-}
-
-impl std::fmt::Display for AudioDevice {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        write!(f, "{}", self.description)
-    }
-}
-
 pub fn enumerate_audio_devices() -> Vec<AudioDevice> {
     let output = Command::new("pw-cli")
         .arg("list-objects")