Merge PR #391 (Vudgekek): macOS CoreAudio Exclusive Mode

Adds CoreAudio Hog Mode integration on macOS so Exclusive Mode actually
takes the device exclusively (previously the macOS path used the
Linux-shaped logic without any kernel-level lock). Pins the CoreAudio
device id through Hog Mode acquisition, sample-rate switching, and
stream creation so System Default doesn't drift mid-session. Linux
behaviour is unchanged.

Review asks (all four resolved by Vudgekek + afonsojramos):

- set_coreaudio_hardware_volume falls back to software volume on Err
  rather than reporting success, so knob-only DACs keep a working
  volume slider via engine.set_volume().
- CoreAudioExclusiveGuard snapshots the device's hardware volume on
  acquire and restores it before releasing Hog Mode, so leaving
  Exclusive Mode returns the DAC to the user's pre-Exclusive level.
  Guard is constructed before the FFI call so partial-acquire failures
  still trigger Drop and release.
- Linux unused-variable warnings on stream_opt and exclusive_guard
  cleaned up via cfg_attr(allow(unused_variables)) on the macOS-only
  guard plumbing. cargo check is clean.
- Silent shared-mode regression on macOS when backend init fails:
  init_device now sets_current_device(None) and returns None on
  exclusive_mode==true so the user sees an error instead of
  unknowingly playing in shared mode.

Bonus: 5f10b4ba adds real CoreAudio device-rate detection for macOS
Shared Mode, +90 lines in backend.rs.

Conflict resolution: single textual collision in
crates/qbz-audio/src/lib.rs (both branches added a pub use line).
Resolved by keeping both, alphabetical order — coreaudio_direct
re-export from pr-391, device_reservation re-export from pre-release.
All other auto-merges verified line-by-line: AlsaDirectStream's
DeviceReservation::acquire flow (alsa_direct.rs:60-97), the macOS
exclusive_mode guard (player/mod.rs:1077-1085 in qbz-player and the
mirror in src-tauri:1024), and CoreAudioExclusiveGuard's volume
snapshot/restore (coreaudio_direct.rs:351,369,373,388) all coexist
correctly because each branch touched different functions.

cargo check clean across all four crates. 9 device_reservation +
3 reserve_dac + 214 src-tauri unit tests pass.

cc

crates/qbz-audio/Cargo.toml

@@ -49,3 +49,4 @@ zbus = { version = "4", default-features = false, features = ["tokio"] }
 [target.'cfg(target_os = "macos")'.dependencies]
 # CoreAudio for device probing, sample rate switching, and (future) hog mode / exclusive mode
 coreaudio-rs = "0.14.1"
+objc2-core-audio = "0.3.2"