Hear a network's deletion at once where no binding declares it #829

After #828, a deleting Network reaches a presence that no hub binding declares only at that presence's next resync, up to a minute after the delete (the wait). The presence controller hears of a change through the hub bindings that declare a presence, and such a presence has none. The hub already carries a signal it can watch: the replicator mirrors every Network onto the hub and deletes the mirror as soon as the Network is deleted (the mirror, its delete). Watching those mirrors, as the controller already watches the interface copies that cells publish (that watch), would mark such a presence's context within seconds.

Proposed scope

• The presence controller watches Networks on the hub, and maps the deletion of a mirror to the hub copies of the contexts in the same namespace whose network label names that network.
• The requeue within the resync stays, for a Network that has no mirror.
• It is a deletion path, so it gets a planted defect per case and a kind run of a deletion with an interface held past its grace.

Non-goals

• No change for a presence that a hub binding declares: it hears the deletion through the binding today.

Acceptance criteria

• On kind, a presence that no binding declares is marked within seconds of its Network's deletion, where it now waits up to a minute
• The hub permission the new watch needs is named, or shown to exist already
• Each case has a test that a planted defect turns red

Related to #664