ban_settings() derived each reason slug via sanitize_key(label), which
strips CJK text to an empty string; the frontend filter then discarded
every reason (including the schema defaults) and fell back to the
hard-coded list. Derive the slug from an ASCII key when possible and
from a stable md5 short hash of the label otherwise, and relax the
frontend filter to the label only. Adds a WP-stub harness that verifies
the defaults path end to end (tests/dev/verify-ban-settings-defaults.php).
