The read. Pass the real file I/O through hold (it returns the
same promise): the slot is held until run() and every held promise has
settled or reached the longer held-I/O deadline.
Optionalsignal: AbortSignal
Aborting it while the read still waits for a slot rejects
with the signal's reason (or an AbortError) and frees its queue place.
Once started, run owns the signal.
Run one OPFS read under the page-wide FIFO concurrency cap.