/// 分转元字符串，千分位分组 + 两位小数，纯整数运算（金额禁 float）。
///
/// 例：299900 → "2,999.00"；100 → "1.00"；0 → "0.00"；负数保留符号。
String yuanFromMinor(int minor) {
  final negative = minor < 0;
  final abs = negative ? -minor : minor;
  final yuan = abs ~/ 100;
  final cents = abs % 100;

  final yuanDigits = yuan.toString();
  final len = yuanDigits.length;
  final buffer = StringBuffer();
  for (var i = 0; i < len; i++) {
    if (i > 0 && (len - i) % 3 == 0) buffer.write(',');
    buffer.write(yuanDigits[i]);
  }

  final centsStr = cents.toString().padLeft(2, '0');
  return '${negative ? '-' : ''}$buffer.$centsStr';
}