AddClusterMember

Add a member at run time (task 115): a Manager whose node name no config.yaml names. The cluster records that it expects node; the node joins when its Manager starts with the existing members (the seed first) in its own clusterNodes — norsk-mgr init --cluster on its host writes that. Ra accepts any joiner carrying the cookie (and the distribution certificate, with TLS on): the row is the cluster’s bookkeeping — the generation check, the follower rejoin, the "whole again" check and the witness hand-over all go by it — not a gate. Idempotent. Forwarded to the primary.