Validate a Legal Entity Identifier (LEI, ISO 17442) with the ISO 7064 mod-97-10 check digits — not just a 20-character regex. Returns {valid, lei (normalized), louPrefix (issuing LOU), checkDigits, reason}. Confirms a counterparty/vendor LEI is well-formed before lookup; pairs with GLEIF data for name + ownership resolution. Structure + checksum only.