RaftRole
The role a node holds at a given instant in Raft's state machine.
Only voters ever hold Leader, Follower, or Candidate. A node listed in ClusterConfig.learners is permanently Learner — it receives log replication from the leader but never votes and never stands for election. A learner can still call RaftNode.propose; the call forwards to the current leader like any non-leader role, and the committed entry replicates back.