chore(seo): redirects 单跳 + root metadata hreflang，i18n 段化收尾

i18n PR (#330) 之后存在两个 SEO 残留问题，本 PR 一并修：

1. next.config.mjs redirects 全部 destination 加 /zh/ 前缀

   现状：destination 都是 /docs/...（不带 locale），用户访问老 URL 流程：
     /docs/CommunityShare/RAG/rag
       → 301 /docs/learn/ai/foundation-models/rag/rag
       → 308 /zh/docs/learn/ai/foundation-models/rag/rag (next-intl middleware)

   两跳 redirect 对 SEO PageRank 不友好（Google 推荐单跳）。把 destination
   直接指向带 /zh/ 前缀的 canonical URL，搜索引擎单跳到位。

   改 46 条规则的 destination，source 保持不带 locale（匹配老外链）。
   英语用户由站内 <LocaleToggle /> 切到 /en/...，cookie 同步偏好。

2. root layout metadata hreflang

   alternates.canonical 之前写死 /，i18n 段化后根路径会被 middleware
   redirect，搜索引擎拿到的 canonical 是个 redirect 目标，不准。
   改为：
   - canonical: /zh（默认 locale 首页）
   - languages: zh-CN / en-US / x-default 三向声明

   注：每个具体 [locale]/page 的 generateMetadata 会覆盖 root metadata 的
   languages，给到该 page 各 locale 的具体 URL。这里 root 只作 fallback。

Author: github-actions[bot]

app/layout.tsx

@@ -53,8 +53,18 @@ export const metadata: Metadata = {
   creator: "longsizhuo",
   publisher: "Involution Hell",
   category: "Technology",
+  // alternates 是 fallback，被 [locale] 段下的 generateMetadata 覆盖。
+  // 默认 canonical 指 /zh（默认 locale 首页），不指 / 因为根路径会被
+  // next-intl middleware 308 redirect，搜索引擎索引到 /zh 更直接。
+  // languages 同时声明 hreflang，让 root metadata 应用到不在 [locale]
+  // 下的路径（如 /sitemap.xml 详情页 fallback 时）也能正确给出语言关系。
   alternates: {
-    canonical: "/",
+    canonical: "/zh",
+    languages: {
+      "zh-CN": "/zh",
+      "en-US": "/en",
+      "x-default": "/zh",
+    },
   },
   robots: {
     index: true,

next.config.mjs

@@ -37,154 +37,161 @@ const config = {
   async redirects() {
     // Option C IA 大重组：按读者意图分 learn / career / community / projects 四大顶层区。
     // 顺序敏感——Next.js 首匹配命中，特殊文件级 + cpp_backend 老名字必须排在 wildcard 前。
+    //
+    // 2026-05 i18n URL 段化（PR #330）：所有 destination 都带 /zh/ 前缀
+    // （默认 locale）。不带前缀的话会先 301 到 /docs/...，next-intl
+    // middleware 再 308 到 /zh/docs/...，对 SEO 是两跳 redirect。直接
+    // destination 写带前缀的 canonical URL 让搜索引擎和用户都单跳到位。
+    // 英语用户由站内 <LocaleToggle /> 切到 /en/docs/...，cookie 同步偏好。
+    // source 保持不带 locale 形式（匹配老 URL）。
     return [
       // ============= 特殊路径（必须在 wildcard 之前） =============
       // CommunityShare/RAG → learn/ai/foundation-models/rag （RAG 文件归 ai 主题）
       {
         source: "/docs/CommunityShare/RAG/rag",
-        destination: "/docs/learn/ai/foundation-models/rag/rag",
+        destination: "/zh/docs/learn/ai/foundation-models/rag/rag",
         statusCode: 301,
       },
       {
         source: "/docs/CommunityShare/RAG/embedding",
-        destination: "/docs/learn/ai/foundation-models/rag/embedding",
+        destination: "/zh/docs/learn/ai/foundation-models/rag/embedding",
         statusCode: 301,
       },
       {
         // 文件名顺手规范化：context_engineering_intro → context-engineering-intro
         source: "/docs/CommunityShare/RAG/context_engineering_intro",
         destination:
-          "/docs/learn/ai/foundation-models/rag/context-engineering-intro",
+          "/zh/docs/learn/ai/foundation-models/rag/context-engineering-intro",
         statusCode: 301,
       },
       // CommunityShare/Geek/leworldmodel → community/papers（paper summary 归社区论文）
       {
         source: "/docs/CommunityShare/Geek/leworldmodel",
-        destination: "/docs/community/papers/leworldmodel",
+        destination: "/zh/docs/community/papers/leworldmodel",
         statusCode: 301,
       },
       // CommunityShare/Amazing-AI-Tools 下两篇分家：tool review 归 tools，paper 归 papers
       {
         source: "/docs/CommunityShare/Amazing-AI-Tools/perplexity-comet",
-        destination: "/docs/community/tools/perplexity-comet",
+        destination: "/zh/docs/community/tools/perplexity-comet",
         statusCode: 301,
       },
       {
         source:
-          "/docs/CommunityShare/Amazing-AI-Tools/prompt-repetition-improves-non-reasoning-llms",
+          "/zh/docs/CommunityShare/Amazing-AI-Tools/prompt-repetition-improves-non-reasoning-llms",
         destination:
-          "/docs/community/papers/prompt-repetition-improves-non-reasoning-llms",
+          "/zh/docs/community/papers/prompt-repetition-improves-non-reasoning-llms",
         statusCode: 301,
       },
       // PPO 强化学习主题 → learn/ai/reinforcement-learning
       {
         source:
-          "/docs/CommunityShare/Personal-Study-Notes/Reinforcement-Learning/ppo",
-        destination: "/docs/learn/ai/reinforcement-learning/ppo",
+          "/zh/docs/CommunityShare/Personal-Study-Notes/Reinforcement-Learning/ppo",
+        destination: "/zh/docs/learn/ai/reinforcement-learning/ppo",
         statusCode: 301,
       },
       // swanlab 之前 test run 已移到 ai/misc-tools/（main commit d6d0a3d），现改到 community/tools
       {
         source: "/docs/ai/misc-tools/swanlab",
-        destination: "/docs/community/tools/swanlab",
+        destination: "/zh/docs/community/tools/swanlab",
         statusCode: 301,
       },
       // cpp_backend 老命名（下划线 / 大驼峰）→ learn/cs/cpp-backend/ (kebab-case)
       {
         source: "/docs/computer-science/cpp_backend/mempool_simple",
-        destination: "/docs/learn/cs/cpp-backend/mempool-simple",
+        destination: "/zh/docs/learn/cs/cpp-backend/mempool-simple",
         statusCode: 301,
       },
       {
         source:
-          "/docs/computer-science/cpp_backend/Handwritten_pool_components/1_Handwritten_threadpool",
+          "/zh/docs/computer-science/cpp_backend/Handwritten_pool_components/1_Handwritten_threadpool",
         destination:
-          "/docs/learn/cs/cpp-backend/handwritten-pool-components/1-handwritten-threadpool",
+          "/zh/docs/learn/cs/cpp-backend/handwritten-pool-components/1-handwritten-threadpool",
         statusCode: 301,
       },
       {
         source:
-          "/docs/computer-science/cpp_backend/Handwritten_pool_components/2_Handwritten_mempool1",
+          "/zh/docs/computer-science/cpp_backend/Handwritten_pool_components/2_Handwritten_mempool1",
         destination:
-          "/docs/learn/cs/cpp-backend/handwritten-pool-components/2-handwritten-mempool1",
+          "/zh/docs/learn/cs/cpp-backend/handwritten-pool-components/2-handwritten-mempool1",
         statusCode: 301,
       },
       {
         source: "/docs/computer-science/cpp_backend/easy_compile/1_cpp_libs",
-        destination: "/docs/learn/cs/cpp-backend/easy-compile/1-cpp-libs",
+        destination: "/zh/docs/learn/cs/cpp-backend/easy-compile/1-cpp-libs",
         statusCode: 301,
       },
       {
         source: "/docs/computer-science/cpp_backend/easy_compile/2_base_gcc",
-        destination: "/docs/learn/cs/cpp-backend/easy-compile/2-base-gcc",
+        destination: "/zh/docs/learn/cs/cpp-backend/easy-compile/2-base-gcc",
         statusCode: 301,
       },
       {
         source: "/docs/computer-science/cpp_backend/easy_compile/3_Make",
-        destination: "/docs/learn/cs/cpp-backend/easy-compile/3-make",
+        destination: "/zh/docs/learn/cs/cpp-backend/easy-compile/3-make",
         statusCode: 301,
       },
       {
         source: "/docs/computer-science/cpp_backend/easy_compile/4_CMake",
-        destination: "/docs/learn/cs/cpp-backend/easy-compile/4-cmake",
+        destination: "/zh/docs/learn/cs/cpp-backend/easy-compile/4-cmake",
         statusCode: 301,
       },
       {
         source: "/docs/computer-science/cpp_backend/easy_compile/5_vcpkg",
-        destination: "/docs/learn/cs/cpp-backend/easy-compile/5-vcpkg",
+        destination: "/zh/docs/learn/cs/cpp-backend/easy-compile/5-vcpkg",
         statusCode: 301,
       },
       // all-projects/ai-town → projects/ai-town （顶层化）
       {
         source: "/docs/all-projects/ai-town",
-        destination: "/docs/projects/ai-town",
+        destination: "/zh/docs/projects/ai-town",
         statusCode: 301,
       },
       // all-projects 裸路径本身也要兜底，防止指 /docs/all-projects 直接 404
       {
         source: "/docs/all-projects",
-        destination: "/docs/projects",
+        destination: "/zh/docs/projects",
         statusCode: 301,
       },
       // CommunityShare / Amazing-AI-Tools index 顶层
       {
         source: "/docs/CommunityShare",
-        destination: "/docs/community",
+        destination: "/zh/docs/community",
         statusCode: 301,
       },
       {
         source: "/docs/CommunityShare/Amazing-AI-Tools",
-        destination: "/docs/community/tools",
+        destination: "/zh/docs/community/tools",
         statusCode: 301,
       },
 
       // ============= Wildcard 顶层区重命名 =============
       // 学科主目录：ai → learn/ai, computer-science → learn/cs
       {
         source: "/docs/ai/:path*",
-        destination: "/docs/learn/ai/:path*",
+        destination: "/zh/docs/learn/ai/:path*",
         statusCode: 301,
       },
       {
         source: "/docs/computer-science/:path*",
-        destination: "/docs/learn/cs/:path*",
+        destination: "/zh/docs/learn/cs/:path*",
         statusCode: 301,
       },
       // 求职场景 jobs/{interview-prep,event-keynote} → career/{interview-prep,events}
       {
         source: "/docs/jobs/interview-prep/:path*",
-        destination: "/docs/career/interview-prep/:path*",
+        destination: "/zh/docs/career/interview-prep/:path*",
         statusCode: 301,
       },
       {
         source: "/docs/jobs/event-keynote/:path*",
-        destination: "/docs/career/events/:path*",
+        destination: "/zh/docs/career/events/:path*",
         statusCode: 301,
       },
       // 项目 all-projects → projects
       {
         source: "/docs/all-projects/:path*",
-        destination: "/docs/projects/:path*",
+        destination: "/zh/docs/projects/:path*",
         statusCode: 301,
       },
       // CommunityShare 分家：其他按主题归 community/*
@@ -193,27 +200,27 @@ const config = {
       //   生成的 slug 映射做字面匹配，单跳 301 到正确拼音 URL。
       {
         source: "/docs/CommunityShare/Language/:path*",
-        destination: "/docs/community/language/:path*",
+        destination: "/zh/docs/community/language/:path*",
         statusCode: 301,
       },
       {
         source: "/docs/CommunityShare/Life/:path*",
-        destination: "/docs/community/life/:path*",
+        destination: "/zh/docs/community/life/:path*",
         statusCode: 301,
       },
       {
         source: "/docs/CommunityShare/MentalHealth/:path*",
-        destination: "/docs/community/mental-health/:path*",
+        destination: "/zh/docs/community/mental-health/:path*",
         statusCode: 301,
       },
       {
         source: "/docs/CommunityShare/Geek/:path*",
-        destination: "/docs/community/dev-tips/:path*",
+        destination: "/zh/docs/community/dev-tips/:path*",
         statusCode: 301,
       },
       {
         source: "/docs/CommunityShare/Amazing-AI-Tools/:path*",
-        destination: "/docs/community/tools/:path*",
+        destination: "/zh/docs/community/tools/:path*",
         statusCode: 301,
       },
     ];