Whether this delta only revises spend already reported for the request.
A correction re-states what an earlier delta already contributed: a
completion replacing its chunks, or a request re-priced under the model it
finally named. New spend is not a correction, even when it arrives on a
request that already has a record, because the tokens themselves are new.
Lets the consumer drop a correction whose subject is gone -- a backend
total has already absorbed that request -- whatever the correction's sign.
A stale positive correction would otherwise re-inflate a display the
total had just settled.
