In v2, both platforms use the test process PID and the node port. The
PID distinguishes parallel tests, and the port distinguishes nodes in
one test. There is only one archiver per node, so retries can safely
reuse the same temporary path.

I also documented that archive commands with concurrent readers should
publish complete files atomically, and warned that the simple cp example
does not. I did not present the helper's mv -f as a production recipe,
because that would contradict the existing requirement not to overwrite
a different pre-existing segment.