feat: implement content reveal mechanism via DOM traversal

Add `addRevealedClasses` to apply `revealed` or `concealed` CSS classes
to article elements based on the current heading hierarchy and
revealed paths. Update `Article` to expose its DOM element via an
`onDom` callback.

src/components/Article.tsx

@@ -21,6 +21,7 @@ export interface ArticleProps {
   onError?: (error: Error) => void;
   class?: string; // 额外的 class 用于样式控制
   scrollToHash?: boolean; // 是否自动滚动到 hash
+  onDom?: (dom: HTMLDivElement) => void;
 }
 
 async function fetchArticleContent(params: {
@@ -64,6 +65,7 @@ export const Article: Component<ArticleProps> = (props) => {
     () => ({ src: props.src, section: props.section }),
     fetchArticleContent,
   );
+  let innerDiv!: HTMLDivElement;
 
   // 解析 iconPath，默认为 "./assets"，空字符串表示禁用
   const iconPrefix = createMemo(() => {
@@ -80,6 +82,8 @@ export const Article: Component<ArticleProps> = (props) => {
 
       // 内容渲染后检查 hash 并滚动
       scrollToHash(location.hash);
+
+      props.onDom?.(innerDiv);
     }
   });
 
@@ -101,6 +105,7 @@ export const Article: Component<ArticleProps> = (props) => {
       <Show when={!content.loading && !content.error && content()}>
         <div
           class="relative"
+          ref={innerDiv}
           innerHTML={parseMarkdown(content()!, iconPrefix())}
         />
       </Show>