Drop the run's ledger — call only once the run is terminal.
Record one more resume of stepKey; resolves to the run's updated counts.
prior is a read-elision hint: the run's current counts as read by
counts() under the SAME per-run lock, with nothing writing in between
(an empty map when the run has none). An I/O-backed ledger uses it to
skip re-reading what the lock guarantees unchanged; implementations with
free reads may ignore it. Omit when no such fresh read exists.
stepKey -> prior resume count for the run; undefined when the run has no recorded resumes. Read BEFORE an increment, the count is the ordinal of the suspension currently being resumed (undefined = first suspension).