Enum Parker 

enum Parker {
    SleepSlice,
    Condvar,
}

How the thread waits when a deadline is armed.

Exactly one implementation ships. SleepSlice is the backend on every platform. Condvar exists because TimerBackend::Condvar and ImpairmentKind::CoarseReleaseTimer are public API and must be reachable by a test: forcing it on Windows produces a genuinely tick-bound wheel — measured p50 lateness 12.256 ms for a 3 ms deadline — which is exactly what that impairment reports, so the fault injection is not a simulation. No #[cfg]: both arms compile on every platform, and the choice is made once, at construction, from MOQTAP_RELEASE_TIMER. This enum is the seam for adding a platform timer without touching the wheel.

Variants

SleepSlice

std::thread::sleep(min(remaining, SLICE)), re-reading the heap between slices.

Condvar

Condvar::wait_timeout for the whole remainder: interruptible everywhere, high-resolution only on Unix.