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.
A curated self-serve menu: operators OFFER <host> / OFFERDEL <n>, anyone
sees OFFERLIST, and a user picks one with TAKE <n> (assigned and applied
at once). Offers are a global event-sourced list (VhostOfferAdded/Removed,
threaded through apply like bots). SETALL/DELALL alias SET/DEL and GROUP
is a no-op reassurance, since the per-account vhost already covers every
grouped nick.
