Function observe_stability 

fn observe_stability(
    replicas: &[(ReplicaHealth, Option<EpochMillis>)],
    now: EpochMillis,
    period_ms: u64,
) -> StabilityObservation

Folds the health of a cluster’s replicas into the gate’s verdict.

Each replica waits for the configured period, capped at its creation age when it hydrated. Missing or invalid creation evidence retains the full period. Replica clocks running behind shorten the wait by their skew.