Validate a product barcode — GTIN-8/12/13/14, UPC-A, EAN-13, or ISBN-10/13 — with the GS1 mod-10 check digit (ISBN-10 uses mod-11). Returns {valid, type, gtin14 (canonical 14-digit form, left-padded), checkDigit, reason}. Normalizes every product identifier to one GTIN-14 key so a product-master/ETL pipeline can dedupe and validate SKUs in one deterministic call instead of doing checksum math in an LLM.

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