website: render the design essays' TeX (math: true, mathjax3 pinned to v4)

design/revertible-effects and design/context-model carry real TeX that was
showing as literal $$ source. markdown: { math: true } enables
markdown-it-mathjax3; pinned ^4.3.2 deliberately — v5 injects a <style> tag
per formula (31 on one page), which Vue's template compiler rejects, so v5
cannot work under VitePress; v4 emits pure SVG (probed: 0 style tags).

website/.vitepress/config/index.ts

@@ -5,6 +5,13 @@ export default defineConfig({
   title: 'DeepSeek Harness',
   description: '插件化 Agent 开发框架',
 
+  // The design essays (design/revertible-effects, design/context-model) carry
+  // real TeX; math: true wires markdown-it-mathjax3 into the pipeline.
+  // markdown-it-mathjax3 is pinned to ^4 (NOT 5.x): v5 injects a <style> tag
+  // per formula, which Vue's template compiler rejects ("Tags with side
+  // effect … are ignored in client component templates"); v4 emits pure SVG.
+  markdown: { math: true },
+
   locales: {
     'zh-CN': zhCN,
   },