Local stays the default and the cheap path; mesh asks each partner for its own
sessions over SSH, live rather than cached, because a stream is true for
minutes and a cached one would be confidently wrong about the only thing the
card exists to say.
Writing down what each endpoint actually guarantees made the places it
didn't obvious — shell arguments reaching a crontab or a bash -c
unescaped, master.conf written without tmp+rename, and conf edits that
could be saved without ever being parsed.