How long to wait for the TUN readiness signal (device creation + stack
init + child-task setup) before treating the listener as failed to
start. Shared between the startup path (meow-app/src/main.rs) and the
config-reload path (routes.rs::spawn_tun_from_raw) so the two don’t
drift. Windows wintun adapter creation plus smoltcp netstack init can
take tens of seconds on slow machines; 5 s proved too aggressive.