Function probe_backoff_ttl 

pub(super) fn probe_backoff_ttl(consecutive_failures: u32) -> Duration

Re-probe interval after consecutive_failures failures. 0 (a healthy probe) uses HEALTHY_TTL; failures grow it exponentially from 30 s, capped at FAILED_BACKOFF_MAX, so a permanently-broken probe settles at one attempt every five minutes instead of every five seconds.