Architecture: Central pricing/adjustment service & financial ledger boundary

[khaira777]

Architecture: Central pricing/adjustment service & financial ledger boundary

Summary

Pricing logic is currently spread across POS, orders and bills. Manual discounts (#359-adjacent), promotions/coupons (#359), combos (#366), tips (#364), gift cards (#363), split checks and refunds (#278) will all interact — a small central pricing/adjustment service and a proper financial transaction/ledger boundary will pay for themselves.

Goals

Order
  ↓
Bill
  ↓
Payments (Cash | Card | Gift card | Loyalty)

Bill adjustments (Manual discount | Promotion | Coupon)

Refunds → reference original payment/bill

Shift → cash in − cash refunds = expected drawer

• A single place that computes prices/adjustments, taking an order snapshot so historical bills are immutable.
• A ledger boundary so promotions/gift cards/tips/refunds don't become four unrelated piles of accounting.
• Incremental: introduce with these features, not a giant "rewrite pricing engine" project.

Related existing work

• Architecture [Needs Review]: Integer minor-unit Money foundation (main/core/money.ts) #285 money (integer minor-unit) — foundation to build on.
• Architecture: Decouple tax engine & payment tender domain boundaries #282 tax engine / tender boundaries — related decoupling.
• Financials: Implement idempotent refund system & fix partial refund over-collection #278 refunds and Feature: Shift lifecycle management, cash drawer reconciliation, & business day close #279 shifts — the ledger consumers.
• Feature [Needs Review]: Append-only EPOS audit event log foundation (main/core/audit.ts) #287 audit event log — ledger events should feed it.

Acceptance criteria

• Order/bill pricing flows through one adjustment module; snapshots stored (see Feature: Promotion engine with coupons as one mechanism #359 order_adjustments).
• Financial transactions write through a ledger boundary (payments, refunds, gift-card movements).
• No behavior change for existing manual discounts/split checks (regression-tested).