- a kind no plugin registered always inserts through the insert-only River
client, so Dispatch and Enqueue no longer fail River's unknown-kind check
while the in-process worker runs
- while a worker runs, such a kind must name a queue no worker serves;
an empty queue, default, scheduled, a configured queue or a registered
job's queue is ErrUnregisteredKindQueue and nothing is written
- README and docs/services/jobs.md describe jobs whose worker ships later
