The particulate effects-lab fixture called `sed -i` with no backup suffix. BSD sed reads the next token as the suffix and then the filename as the script, so it died with "invalid command code f" and the corpus-engine fixture gate exited non-zero on macOS. `./tools/check.sh --docs` therefore could not complete on a clean tree. Use the `-i.bak` + `rm` idiom the same file already uses two hundred lines later. Defense: tooling-only portability fix. No game behavior, wiki authority, or save format is touched, so no design amendment is owed.