Function arm_at 

pub(crate) fn arm_at(at: Instant) -> Deadline

Register at with the process-wide wheel.

The sole lazy-initialisation trigger for the release thread. When at <= Instant::now() it returns an already-cancelled Deadline without constructing the wheel, so Delay { by: 0 }, a delay swallowed by the monotonic clamp, and every session that never delays all leave crate::instrument::release_timer_started false.