feat: Implement Phase 6 Shopping and Checkout

- Introduced authenticated shopping state and atomic checkout process without creating orders or payments.
- Reused existing components from previous phases including identities, addresses, catalogues, pricing, and inventory.
- Developed cart architecture to support one active cart per user with unique items per variant.
- Implemented dynamic cart pricing with various precedence rules for promotions and coupons.
- Created a wishlist feature that exposes only visible active products without reserving inventory.
- Integrated inventory validation to ensure availability of items before adding to cart.
- Developed a checkout architecture that locks the cart, revalidates pricing and shipping, and reserves inventory.
- Added shipping zones, methods, and rates with configurable options for international shipping and duty/tax boundaries.
- Implemented checkout snapshots to capture immutable checkout details.
- Introduced idempotency for checkout requests to prevent duplicate processing.
- Added functionality for coupon integration and business checkout validation.
- Established permissions for managing shipping zones, methods, and rates.
- Created comprehensive unit tests covering various aspects of the shopping and checkout processes.
- Added cron job for handling checkout expiry reconciliation.
- Created migration script to set up new database tables and constraints for carts, checkout sessions, and shipping.

app/services/pricing/money.js

@@ -2,4 +2,5 @@ const parse = value => { const match = String(value).match(/^(\d+)(?:\.(\d{1,2})
 const format = cents => `${cents / 100n}.${String(cents % 100n).padStart(2, "0")}`;
 const subtractFloor = (a, b) => format(parse(a) > parse(b) ? parse(a) - parse(b) : 0n);
 const percentOff = (amount, percent) => { const [whole,fraction=""] = String(percent).split("."); const basis = BigInt(whole) * 100n + BigInt(fraction.padEnd(2,"0")); return format(parse(amount) * basis / 10000n); };
-module.exports = { parse, format, subtractFloor, percentOff };
+const add=(...values)=>format(values.reduce((sum,value)=>sum+parse(value),0n));const multiply=(amount,quantity)=>format(parse(amount)*BigInt(quantity));
+module.exports = { parse, format, subtractFloor, percentOff, add, multiply };