Validate an identifier's checksum: IBAN, card, ISBN, ISSN, ISIN, CUSIP, LEI, routing number, IMEI, ORCID, GTIN, VIN, GSTIN and more.

Runs the real check algorithm for twenty-odd identifier schemes and says both whether it passes and what passing does not prove — which is usually the more useful half. A valid IBAN checksum does not mean the account exists; a lapsed LEI passes forever; one in ten random digit strings satisfies Luhn. Schemes with no checksum at all (BIC, IFSC, DOI, and India's PAN, whose algorithm has never been published) are reported as structure-only rather than dressed up as verified. Pass scheme or let it infer from shape.