Codex 嚴謹 diff 審查官：正確性、簡化、安全三軌過濾

[TRACK A] [CONFIDENCE: high] src/cart.ts:42 What: applyDiscount 在 items 為空陣列時回傳 NaN Why it matters: 空購物車結帳會把 NaN 寫進訂單金額，下游付款端會拋錯 Fix: const subtotal = items.reduce((a, i) => a + i.price, 0); if (items.length === 0) return 0; [TRACK B] [CONFIDENCE: medium] src/cart.ts:55 What: 手寫的 round-to-2 與既有 utils/money.ts 的 toCents() 重複 Why it matters: 兩套四捨五入邏輯會在邊界分歧（0.005） Fix: import { toCents } from '../utils/money' Track C: no issues found VERDICT: merge-after-fixes。先修 src/cart.ts:42 的空陣列 NaN，這會直接影響結帳。