Merge branch 'mm/lib-httpd-cgi-safe' into jch

CGI helper scripts used by HTTP-related test scripts have been updated
to use atomic filesystem operations, preventing race conditions when
Apache handles concurrent requests.

* mm/lib-httpd-cgi-safe:
  t/README: document writing concurrency-safe helpers
  t/lib-httpd: make http-429 first-request check atomic
  t/lib-httpd: fix apply-one-time-script race under concurrent requests

diff --cc t/README
Simple merge

diff --cc t/lib-httpd.sh
Simple merge

diff --cc t/meson.build
Simple merge