Opt-in flag for the per-process DAC reservation lifetime (Lifetime B
in the spec). Defaults to false; serde default ensures legacy settings
JSON files round-trip cleanly without the field. Task 5 will read this
flag in AppState startup and the v2_set_reserve_dac_while_running
Tauri command.
Strictly additive: no existing field changed, no behaviour change for
users with the flag at its default (false).
cc
vicrodh
a1a056c4b9