Function classify_collection_readiness 

fn classify_collection_readiness<'a, I>(
    replicas: I,
    target_replica_ids: &BTreeSet<ReplicaId>,
    reference_replica_ids: &BTreeSet<ReplicaId>,
    allowed_lag: Option<Timestamp>,
) -> CollectionReadiness
where
    I: Iterator<Item = (ReplicaId, &'a ReplicaCollectionState)> + Clone,

Classifies one collection’s readiness over every replica hosting it.

Hydration and sufficient progress must belong to the same target replica. No reference replicas yields MIN, while a completed reference yields the empty frontier. Those cases must remain distinct.