The task a node is reached at before it has said anything. A convention,
because nothing else could be: an address is only learned by being told, so
a node just dialled is unreachable unless one address on it is known already.
('host gate). The only thing that opens a link, so the only thing that
names one: the counter needs neither sharing nor an atomic, which is why a
listener’s accepts come back here instead of becoming links where they
arrive. Numbering starts past the ids a launcher may have registered before
the loop began (crate::world::World::link).