Deep-review round 5. The assertion behind `subtitles._exists` was only red where pathlib re-raises
ENAMETOOLONG: on Linux it does, on Windows the same stat fails with ERROR_INVALID_NAME, which
pathlib's `exists()` already swallows — so the assertion passed with the fix reverted. It forces the
errno now instead of relying on the filesystem. (The gate's pytest lane is the Linux container, so
the branch was never actually unguarded; the test was just dishonest about why it passed.)
peter
33fddacee2