CLOSED → OPEN → HALF-OPEN → CLOSED

Implement CircuitBreaker(threshold=3, cooldown=30.0) with allow(now) and record(now, ok): - starts CLOSED; allow returns True - each record(now, False) in CLOSED increments failures; reaching threshold trips state to OPEN and stamps _opened_at = now - while OPEN, allow returns False until now - _opened_at >= cooldown, then switches to HALF-OPEN and returns True exactly once (the probe) - record(now, True) in any state: resets failures and closes the circuit - record(now, False) in HALF-OPEN: re-opens (stamp _opened_at = now) now is always a float parameter — no time functions inside.