SET <account> <host> [duration] assigns a vhost that lapses after the
duration (e.g. 30d); a Vhost carries an optional expiry and the store hides
an expired one lazily (like a suspension), so it stops applying on identify/
ON without a sweep. LIST flags temporary vhosts.
New HostServ pseudo-service assigns and applies vhosts. A vhost is a typed
Option<Vhost>{host,setter,ts} on the account (VhostSet/VhostDeleted events),
applied to the displayed host on identify and re-applied by ON; OFF restores
the normal host for the session. Operators SET/DEL (applied at once to
online sessions) and LIST; hosts are validated as dot-separated labels.
New SetHost NetAction -> InspIRCd ENCAP <target-sid> CHGHOST <uid> <host>.
