Fetch full detail for one canonical product by product_id (UUID) or GTIN barcode, including every per-chain listing (chain-specific item code, display name, and package size). Use after search_products to confirm identity, or directly when the GTIN is already known. Each listing carries orderable: false means no delivery or pickup storefront prices it, so it cannot be bought through this API. A listing is catalogue identity, not availability: never present an orderable: false chain as somewhere the shopper can buy. When every listing is orderable: false the product is not purchasable right now, whatever its chains suggest.