fork.mode: 'finalized-only' mapped a block above the dataset’s finalized head.
That mode is a promise that the route publishes nothing a fork could orphan, and a signal cannot be
retracted once it is on the wire.
Fix — read the finalized stream, withhold the signal until its block finalizes, or switch the
route to fork.mode: 'boundary' and publish a compensating boundary message on a fork.