iter: e02 新增 stepper 步骤条组件（三态流转 + active 信号双向绑定 + clickable 回跳）

packages/skin/ui/src/index.ts

@@ -24,6 +24,7 @@ export { pagination } from './pagination.js';
 export { spinner } from './spinner.js';
 export { progress } from './progress.js';
 export { tabs } from './tabs.js';
+export { stepper } from './stepper.js';
 export { modal } from './modal.js';
 export { drawer } from './drawer.js';
 export { form, formItem } from './form.js';