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/mod.rs

@@ -66,9 +66,35 @@ pub mod pan;
 pub mod pipewire_impl;
 #[cfg(windows)]
 pub mod cpal_impl;
+#[cfg(unix)]
 pub mod pw_cli;
 pub mod recorder;
 
+/// A selectable audio device for the input/output pickers. `name` is the stable
+/// identifier the backend uses to request the device (`target_node`);
+/// `description` is the human-facing label shown in the UI. The two may be equal
+/// (cpal/WASAPI) or differ (PipeWire node name vs. description).
+#[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)
+    }
+}
+
+// Enumerate audio input/output devices for the pickers (sorted by description),
+// returning the same `AudioDevice` shape regardless of platform: PipeWire
+// (`pw-cli`) on Linux, cpal/WASAPI on Windows.
+#[cfg(unix)]
+pub use pw_cli::enumerate_audio_devices;
+#[cfg(windows)]
+pub use cpal_impl::enumerate_audio_devices;
+
 /// The audio backend implementation for the current platform.
 ///
 /// The whole app constructs and threads this alias (via
@@ -77,8 +103,7 @@ pub mod recorder;
 /// [`AudioBackend`] trait, which is the only interface the core talks to.
 ///
 /// - Linux/Unix → PipeWire ([`pipewire_impl::PipeWireBackend`]).
-/// - Windows → cpal/WASAPI ([`cpal_impl::CpalBackend`]); a no-op stub until the
-///   Phase 1 capture/playback implementation lands.
+/// - Windows → cpal/WASAPI ([`cpal_impl::CpalBackend`]).
 #[cfg(unix)]
 pub type PlatformAudioBackend = pipewire_impl::PipeWireBackend;
 #[cfg(windows)]