Lifetime B from the design spec: a long-lived DeviceReservation guard
held in AppState, gated by the reserve_dac_while_running setting.
Acquired at startup if the persisted flag is true and the selected
device targets a single ALSA card; reapplied when the user picks a
different DAC; released on toggle-off, on a switch to a non-card
device, or AppState drop.
DB persistence: ALTER TABLE migration adds the column, SELECT and
reset_all are extended, set_reserve_dac_while_running setter parallels
the established pattern for boolean audio settings.
Tauri commands (V2 cleanroom):
- v2_set_reserve_dac_while_running(enabled) - persist + apply
- v2_get_dac_reservation_status() -> DacReservationStatus
Status enum mirrors the four cases in the spec: Inactive (toggle off,
or non-card-specific device), Active (we hold the bus name), Contested
(another app holds the device at higher priority - guard is None but
the toggle is on), Unavailable (D-Bus unreachable, degraded guard).
v2_set_audio_output_device gains an AppState dependency so DAC changes
trigger a reservation handover when the toggle is on.
UI surfaces (Settings mirror + HiFi Wizard) land in Tasks 7-8.
cc
vicrodh
4797fec939