Build a coupon and discount engine

• A cart holds line items (sku, category, unit price in minor units, quantity); applyCoupons(cart, codes) returns the discounted lines, applied discounts and the final total
• Coupon types: percentage off, flat amount off, and buy-one-get-one (BOGO) on a sku or category; a percentage is capped by an optional maximum amount
• Eligibility conditions: minimum cart value, applicable categories or skus, valid date window, first order only, and user segment
• Usage limits: total redemptions and per user redemptions; a redemption counts only when an order is confirmed, and a failed order releases it
• Stacking rules: each coupon has a priority and a flag saying whether it can combine with others; the engine applies them in priority order and rejects or skips incompatible ones with a clear reason
• The total never goes below zero, discounts never exceed the line price, and each applied discount is shown in a result breakdown

Only after every requirement works.

• Per-item discount allocation across lines for refunds
• Rules loaded from JSON configuration
• A "best deal" mode that picks the combination with the largest saving