Cheap structural IBAN validation: ISO 13616 checksum + per-country structure/length. Returns valid plus, on rejection, a

Cheap structural IBAN validation: ISO 13616 checksum + per-country structure/length. Returns valid plus, on rejection, a granular error_type (invalid_checksum, invalid_length, invalid_structure, invalid_country), the country and the format spec. Offline (schwifty, MIT); no network or keys. A cheap-first entry before the fuller /bank/verify fusion route.