fix(v2): money 去 float 进制 + 溢出防护 + 补 Format/负数/溢出测试

- money.go: 用纯 int64 循环乘法替代 int64(math.Pow10(exp)) 计算进制因子
- Parse: whole*mult+frac 前加溢出检查(math.MaxInt64 仅作整型常量参与比较,无 float 参与)
- money_test.go: 补 Format 未知币种错误分支(errors.Is ErrUnknownCurrency)、
  Parse 溢出(整数部分过大)、负数往返测试

internal/money/money.go

@@ -24,6 +24,16 @@ func Exponent(currency string) (int, bool) {
 	return e, ok
 }
 
+// pow10Int64 computes 10^exp as a pure int64 (no float), matching the
+// currency exponents above which never exceed a handful of digits.
+func pow10Int64(exp int) int64 {
+	mult := int64(1)
+	for i := 0; i < exp; i++ {
+		mult *= 10
+	}
+	return mult
+}
+
 // Format renders minor units as a decimal string, trailing zeros trimmed.
 func Format(minor int64, currency string) (string, error) {
 	exp, ok := Exponent(currency)
@@ -37,7 +47,7 @@ func Format(minor int64, currency string) (string, error) {
 	if neg {
 		minor = -minor
 	}
-	div := int64(math.Pow10(exp))
+	div := pow10Int64(exp)
 	whole := minor / div
 	frac := minor % div
 	s := fmt.Sprintf("%d.%0*d", whole, exp, frac)
@@ -81,7 +91,11 @@ func Parse(s, currency string) (int64, error) {
 			return 0, fmt.Errorf("money.Parse frac %q: %w", s, err)
 		}
 	}
-	minor := whole*int64(math.Pow10(exp)) + frac
+	mult := pow10Int64(exp)
+	if mult > 0 && whole > (math.MaxInt64-frac)/mult {
+		return 0, fmt.Errorf("money.Parse: %q overflows int64 for %s", s, currency)
+	}
+	minor := whole*mult + frac
 	if neg {
 		minor = -minor
 	}