Function isr_candidate_selected 

pub fn isr_candidate_selected(facts: IsrCandidateFacts) -> bool

Decide whether one candidate belongs in the next ISR proposal.

The leader always stays, and a member that is no longer assigned leaves. An in-sync follower stays exactly while Kafka’s ReplicaState.isCaughtUp holds, which is Partition.getOutOfSyncReplicas: a follower that keeps fetching but never catches up leaves once its last caught-up time is older than the bound. An out-of-sync follower joins exactly when Kafka’s Partition.needsExpandIsr would add it: it is ISR-eligible (replica_isr_eligible) and in sync by Partition.isFollowerInSync.