Support per-code combination rules and free-gift discount code

Request to add Advanced Discount Code features: 1) Free-gift / free-product module so a code (e.g., 'matchabag') can add a gift product to the cart when applied. 2) Per-code combination rules allowing merchants to specify which discount codes may or may not be combined with each other (e.g., BF15 and BF20 mutually exclusive; 'matchabag' combinable with BF25 OR BF30 but never with both at the same time). 3) Ability to express these rules across campaigns (not only by discount category) and ensure deterministic enforcement at checkout. Target merchant: Shopify Plus stores. Include UI for configuring pairwise code relationships and testing behavior.