Dual-channel opening: consecutive failures (CHS-compatible, no streak
bump) plus windowed failure rate (two 30s buckets, min_calls guard).
429s bypass both channels as backpressure, and a probe hitting 429
releases instead of holding the lease. Open duration doubles per
rate/probe reopen up to max_cooldown_s, decaying after stable CLOSED.
record_success gains count_attempt so bad-result successes stay out of
the window.
iomgaa