Initial commit

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lib/extension/map_extension.dart

@@ -0,0 +1,5 @@
+/// 可空 Map 扩展
+extension MapNullableX on Map? {
+  /// 长度，null 视为 0
+  int em() => this?.length ?? 0;
+}