feat(search): 站内搜索/RSS/sitemap/Artalk 懒挂载

- astro-pagefind：构建期索引；SearchDialog 首开懒加载 + 200ms 防抖，
  入口=Header 钮 + 键盘 /（P0 键位集补全）
- @astrojs/rss：posts 全量（draft 排除），双语分流留 Phase 3
- @astrojs/sitemap：sitemap-index 产出
- ArtalkComments：IntersectionObserver 进入视口才拉 Artalk.js，
  pageKey=/archives/{slug}/（中英共键，评论不分裂）

验证: pnpm astro build exit 0；dist/pagefind 目录 + rss.xml +
      sitemap-index.xml 三件断言过

Spec: 2026-10-24_add-blog-skeleton
Co-authored-by: GLM-5.3

src/layouts/BaseLayout.astro

@@ -12,6 +12,7 @@ import Header from '../components/Header.astro';
 import Footer from '../components/Footer.astro';
 import ThemeScript from '../components/ThemeScript.astro';
 import Keyboard from '../components/geek/Keyboard.astro';
+import SearchDialog from '../components/SearchDialog.astro';
 import BuildInfo from '../components/geek/BuildInfo.astro';
 import { ClientRouter } from 'astro:transitions';
 
@@ -47,5 +48,6 @@ const { title, description = 'wait4x 的博客：阅读 · 技术 · 摄影', tr
       <BuildInfo slot="build-info" />
     </Footer>
     <Keyboard />
+    <SearchDialog />
   </body>
 </html>