Zero-dependency implementation of the German banking industry's account number check digit methods ("Prüfzifferberechnungsmethoden in der deutschen Kreditwirtschaft"), as catalogued by the Deutsche Bundesbank. Validates whether a Kontonummer is structurally consistent for a given method, independently of any IBAN or BLZ master-data lookup. Deliberately standalone: does not depend on iban-commons, so it can be used on its own by callers that only have a BLZ ...