Twice in a long session the suite came back with one failure in this file, and
neither run could be reproduced afterwards — not by seed, not by repetition.
The one thing this spec has that the other 39 examples do not is a fixed path
under `tmp/`, which anything else on the machine can also be inside.
`Dir.mktmpdir` instead, so two runs cannot see each other's folders at all. If
a failure survives this, it is about the code and not about the directory.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>