Address review feedback from the macOS Exclusive Mode PR.
Pin the resolved CoreAudio output device before acquiring Hog Mode so
System Default cannot drift between Hog Mode acquisition, sample-rate
switching, and stream creation.
Fall back to software volume when CoreAudio hardware volume is unavailable,
which keeps the volume slider usable on knob-only DACs.
Snapshot and restore CoreAudio hardware volume around Exclusive Mode so
leaving Exclusive Mode does not leave the output device at QBZ's last
hardware volume.
Refuse shared-mode fallback when macOS Exclusive Mode stream creation
fails, so the app does not silently play in non-exclusive mode while the UI
still reports Exclusive Mode.
Clean up Linux unused-variable warnings from the macOS-only guard plumbing.
Afonso Jorge Ramos
88e072a4ac