Advance a circuit breaker

Implement breaker_step(state, event, now, threshold=2, cooldown=5). State has mode (closed/open/half_open), failures, until, and probe. The valid initial state is closed with zero failures, until 0, probe False. A request in closed is admitted. In open, only now >= until admits a request and moves to half_open with probe True. Further half_open requests are denied. A success outcome in closed or half_open resets the initial state. A failure increments failures; reaching threshold or failing a half-open probe opens until now+cooldown and clears probe. Reject outcomes while open. Return (new_state, admitted) for requests and (new_state, None) for outcomes without mutating input. Outcomes in closed are assumed to correspond to admitted requests; this lab does not track concurrent request IDs.