migrate: Next.js 16 → Astro 7

[PaloMiku]

文档站框架迁移，内容零搬迁（53 个 mdx + 11 个 meta.json 原位保留）。

• 构建栈换成 astro 7.3.5 + fumadocs 16.15.18，Tailwind 4 走 @tailwindcss/vite
• 内容管线改为 Astro Content Collections + 手写 StaticSource，删除 source.config.ts
• 新增 src/lib/astro-jsx.ts 与 src/components/{card,accordion}.tsx 兼容层， 处理 Astro 7 下 JSX-as-prop 与 Radix Context 跨 slot 的问题
• 路由层由 src/app/** 换为 src/pages/**；/sitemap.xml 改为手写端点
• CI/部署切到 dist/，行尾统一 LF，移除 11 个已废弃依赖
• 首页文案按 Mix Space 实际技术栈修正（Core 用 NestJS，主题用 Next.js）
• .gitignore 去掉 Next/yarn 残留与无效的精确文件名，修正会误提交构建产物的规则

验证：build/check/lint/pangu 全绿；URL 集合与旧站零差异；断链阻塞 0。

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