Earlier test used '/proc/0/...' as the unwritable dir. On Windows this
resolved to 'C:\proc\0\...' and mkdir({recursive: true}) silently created
it — the silent code path was never exercised, plus filesystem side-effect
leaked outside the test tmpdir.
Replace with a path that points to an existing file (mkdir on a file path
fails on every platform). Also add a companion test that confirms silent
is opt-in: without {silent: true}, the same fs failure DOES throw.
7 tests pass (was 6).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
