benchmark/Makefile has always been local-only, so the numbers it produces
were never recorded anywhere. Wire it up to a manual workflow so a run can
be kicked off and its output kept in the job summary.
This does not gate anything: it reports absolute throughput for the
current ref, with Crow v1.3.1 alongside for reference. Absolute req/s is
only comparable against other runs on the same runner type, which is why
the ref, runner and load parameters are recorded next to the numbers.
Use benchmark-ab instead when the question is whether a specific change
made things faster; comparing absolute numbers across runs cannot answer
that.
Linux and macOS only. Windows needs benchmark/Makefile rewritten first,
since it relies on nc, & and kill.
