The controller declares each hive's wanted agent set; the hive reads its own
key. Deliberately not a mirror of `status`, which the module documents as a
table: authored by the controller rather than the hive, DECLARED rather than
observed, and — the row that decides how it must be built — unrecoverable if
the store is lost, where status regenerates because every hive republishes
what it is.
Two open functions rather than one shared `open_or_create`. `status` shares its
constructor because either end may legitimately arrive first on a fresh swarm;
here the writer is single and known, so the hive gets a read-only open
returning `Option` and holds no grant to create the bucket. Its absence is the
ordinary pre-publication state, not an error a hive could fix.
Absence is also not a deletion order, which the module says at the place an
implementer will meet it: swarm-side lifecycle does not yet cover agents that
predate it, so a hive finding no key has learned nothing about what it runs —
converging to an empty set would tear those down.
No consumer yet. This is the half that is invariant under the scope semantics
and cadence still being decided.
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