Add ProcessAddPeer/ProcessRemovePeer to JetStreamMetaGroup for
peer-driven stream reassignment. Includes AddKnownPeer/RemoveKnownPeer
tracked in a HashSet, RemovePeerFromStream, RemapStreamAssignment with
replacement-peer selection from the known pool, and DesiredReplicas on
RaftGroup for under-replication detection. Go ref: jetstream_cluster.go:2290-2439.
