The worker ack file is what the gateway's exists()-then-read loop keys on;
the test drives the production entry point _run_external_worker_payload and
asserts the ack is never observable while its body is still being written
(red on origin/main: O_CREAT made the empty file visible before json.dump),
and that no temp residue is left beside it.
Live probe (real `python -m cron.scheduler --external-worker-file` on a
scratch HERMES_HOME, tight parent-style poller): origin/main 3/3 runs read
an empty ack -> "unreadable acknowledgement"; this head 5/5 read a full ack.
Refs #116164 (form 1), #107184. Salvages #107678 (@keenvc).
