Codex 為既有程式補測試：鎖住現有行為、專攻邊界、零行為改動

// src/pricing.test.ts — characterization tests for calcTax() test('applies 5% for standard rate', () => expect(calcTax(100,'std')).toBe(105)) test('returns input unchanged for unknown rate code', () => expect(calcTax(100,'???')).toBe(100)) // NOTE: current behavior may be a bug — locked for now test('negative amount yields negative tax (not clamped)', () => expect(calcTax(-100,'std')).toBe(-105)) test('throws on null amount', () => expect(() => calcTax(null,'std')).toThrow(TypeError)) $ vitest run src/pricing.test.ts ✓ 4 passed Suspected bugs (not fixed): 負數金額未夾零、未知 rate code 靜默回傳原值不報錯。 Coverage gaps left: 未測 currency rounding（需 i18n 環境、另案處理）。