addLink
Admit a Connection to a peer that dialed in after construction.
Exchanges the mesh preamble to learn the remote PeerId, dedups against existing links using the same canonical rule as construction (see meshSeam), updates peers, and launches the link's read loop. Suspends until the preamble exchange completes.
Parameters
conn
A fresh, unread Connection. The mesh wraps it with singleCollection before reading, so the preamble read and the read loop share ONE collection of Connection.incoming — a cold, single-collection conn (a stream fabric's framed()) works as well as a hot channel-backed one, exactly as construction does.