refactor: Markdown 管线迁移到 Sätteri（Phase 1 + Phase 2）

把 markdown.processor 从 unified() 换成 Astro 7 的默认管线 satteri()，
并把原来 12 个 remark/rehype 插件逐一移植到 Sätteri 的 MDAST/HAST 插件模型。

内置能力直接替代（零代码）
- remark-math      → features.math
- remark-directive → 未启用（见下）
- rehype-slug      → satteriHeadingIdsPlugin()

新增 src/plugins/satteri/（6 个插件，约 200 行）
- reading-time.mjs    after 钩子写 ctx.data.astro.frontmatter.minutes/words
- excerpt.mjs         取第一段写 frontmatter.excerpt
- shift-headings.mjs  工厂按 fileURL 判断，只对 posts 下的文章生效
- sectionize.mjs      remark-sectionize 的等价实现（递归分组 + data.hName）
- autolink-headings.mjs  标题锚点，输出与 rehype-autolink-headings 逐字一致
- image-lightbox.mjs  上一版 rehype 图片灯箱的 HAST 版

删除（已迁移或本就无效）
- remark-reading-time / remark-excerpt / remark-shift-headings /
  remark-directive-rehype / rehype-component-admonition /
  rehype-image-lightbox —— 共 6 个旧插件文件
- 未启用 features.directive：内容里的 `::: tip`（冒号后有空格）和
  `> [!tip] Tip` 在迁移前后都渲染成普通段落/引用，属于死语法；
  保持现状以免升级顺手把它激活。

行为对齐
- smartPunctuation: { dashes: false } —— Astro 的 unified 管线会把直引号
  排成弯引号但不转换 ---，Sätteri 默认会转，显式关掉 dashes 才能逐字一致。

验证（与 unified() 构建逐页对比，41 页）
- 正文 HTML 完全一致 35 页；把 HTML 实体和表格对齐写法归一化后，
  只剩 3 页有差异：1 页是 KaTeX 尚未迁移（见下），2 页是中文/emoji 旁的
  引号方向（Sätteri 的结果更正确）。
- meta description 仍由 excerpt 正确生成；阅读时长与基线一致。
- astro check：1 error / 0 warnings，与迁移前完全相同（原有 footer.astro 报错）。
- pnpm build 41 页通过；dev server 正常。

尚未迁移
- KaTeX：Sätteri 的 math 只负责解析，不渲染。数学公式目前落成
  <code class="language-math">，是可见的回归，Phase 3 处理。
- package.json 里 remark-*/rehype-* 依赖暂未清理，等 Phase 3 确定后再收。

astro.config.mjs

@@ -5,30 +5,17 @@ import mdx from "@astrojs/mdx"
 import { pluginCollapsibleSections } from "@expressive-code/plugin-collapsible-sections"
 import { pluginLineNumbers } from "@expressive-code/plugin-line-numbers"
 import expressiveCode from "astro-expressive-code"
-import rehypeAutolinkHeadings from "rehype-autolink-headings"
-import rehypeComponents from "rehype-components"
-import rehypeKatex from "rehype-katex"
-import rehypeSlug from "rehype-slug"
-import remarkDirective from "remark-directive"
-import remarkGithubAdmonitionsToDirectives from "remark-github-admonitions-to-directives"
-import remarkMath from "remark-math"
-import remarkSectionize from "remark-sectionize"
-import { remarkShiftHeadings } from "./src/plugins/remark-shift-headings.mjs"
-import { AdmonitionComponent } from "./src/plugins/rehype-component-admonition.mjs"
-import { parseDirectiveNode } from "./src/plugins/remark-directive-rehype.js"
-import { remarkExcerpt } from "./src/plugins/remark-excerpt.js"
-import { remarkReadingTime } from "./src/plugins/remark-reading-time.mjs"
 import { pluginLanguageBadge } from "./src/plugins/expressive-code/language-badge"
 import { pluginCustomCopyButton } from "./src/plugins/expressive-code/custom-copy-button"
-import { unified } from "@astrojs/markdown-remark"
-import { rehypeImageLightbox } from "./src/plugins/rehype-image-lightbox.mjs"
+import { satteri, satteriHeadingIdsPlugin } from "@astrojs/markdown-satteri"
+import { excerpt } from "./src/plugins/satteri/excerpt.mjs"
+import { readingTime } from "./src/plugins/satteri/reading-time.mjs"
+import { sectionize } from "./src/plugins/satteri/sectionize.mjs"
+import { shiftHeadings } from "./src/plugins/satteri/shift-headings.mjs"
+import { autolinkHeadings } from "./src/plugins/satteri/autolink-headings.mjs"
+import { imageLightbox } from "./src/plugins/satteri/image-lightbox.mjs"
 import { site } from "./src/data/site"
 
-const createAdmonitionComponent = (type) => (properties = {}, children = []) => {
-  const normalizedChildren = Array.isArray(children) ? children : []
-  return AdmonitionComponent(properties, normalizedChildren, type)
-}
-
 export default defineConfig({
   srcDir: "./src",
   output: "static",
@@ -88,56 +75,24 @@ export default defineConfig({
   ],
   compressHTML: true,
   markdown: {
-    processor: unified({
-      remarkPlugins: [
-        remarkMath,
-        remarkReadingTime,
-        remarkExcerpt,
-        remarkShiftHeadings,
-        remarkGithubAdmonitionsToDirectives,
-        remarkDirective,
-        remarkSectionize,
-        parseDirectiveNode,
+    // Astro 7 的 Sätteri 管线。以下插件是 unified 时期 remark/rehype 插件的对应移植，
+    // 见 src/plugins/satteri/。
+    //
+    // 注意：math 只负责「解析」，渲染成 KaTeX 需要额外的 HAST 插件 —— 尚未迁移。
+    processor: satteri({
+      // dashes: false 是为了和迁移前的输出逐字一致：Astro 的 unified 管线
+      // 会把直引号排成弯引号，但不转换 --- （Sätteri 默认会转成破折号）。
+      features: { math: true, smartPunctuation: { dashes: false } },
+      mdastPlugins: [
+        readingTime,
+        excerpt,
+        shiftHeadings(),
+        sectionize(),
       ],
-      rehypePlugins: [
-        rehypeKatex,
-        rehypeSlug,
-        [
-          rehypeComponents,
-          {
-            components: {
-              note: createAdmonitionComponent("note"),
-              tip: createAdmonitionComponent("tip"),
-              important: createAdmonitionComponent("important"),
-              caution: createAdmonitionComponent("caution"),
-              warning: createAdmonitionComponent("warning"),
-            },
-          },
-        ],
-        [
-          rehypeAutolinkHeadings,
-          {
-            behavior: "append",
-            properties: {
-              className: ["anchor"],
-            },
-            content: {
-              type: "element",
-              tagName: "span",
-              properties: {
-                className: ["anchor-icon"],
-                "data-pagefind-ignore": true,
-              },
-              children: [
-                {
-                  type: "text",
-                  value: "#",
-                },
-              ],
-            },
-          },
-        ],
-        rehypeImageLightbox,
+      hastPlugins: [
+        satteriHeadingIdsPlugin(),
+        autolinkHeadings,
+        imageLightbox,
       ],
     }),
   },