admit

Admit a (re-)connecting client whose local link into this server is link.

Registers the local spoke first so that a frame which races in across the core — from a peer server that already learned of this attachment — finds the spoke waiting, then publishes client → self into the replicated directory so the rest of the core re-homes its routing here. On a failover this is the whole of the overlay consequence: the directory write supersedes the client's old (dead-server) entry under last-writer-wins.