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

package.json

@@ -10,9 +10,12 @@
     "astro": "astro"
   },
   "dependencies": {
+    "@astrojs/rss": "^4.0.19",
+    "@astrojs/sitemap": "^3.7.4",
     "@fontsource/jetbrains-mono": "^5.3.0",
     "@fontsource/noto-sans-sc": "^5.3.1",
     "astro": "^7.3.8",
+    "astro-pagefind": "^2.0.1",
     "exifr": "^7.1.3",
     "lxgw-wenkai-screen-webfont": "^1.7.0",
     "sharp": "^0.35.5"