feat(i18n): /en/ 双语骨架——路由/切换/hreflang/回退/AI 标注

- astro i18n（zh 默认无前缀，en /en/）+ locale.ts 路径工具
- Header 语言切换（同路径跨前缀跳转，预构建毫秒级）
- /en/index + /en/archives/{slug}（postsEn 优先，缺译回退 zh 正文
  + AiTranslatedBadge 虚线标注 TMT）；en RSS（标题回退同策略）
- BaseLayout hreflang 双向 + x-default 指向 zh 源
- posts-en 集合（postsSchema.extend locked 润色锁）
- 坑：ts 端点文件误带 --- 开头会以 undefined:2:7 报 Unexpected token

验证: astro build 18 页 exit 0；/en/ 六文章路由+首页+rss 产物断言过；
      zh 侧零回归（11 页不变）；hreflang 抽查 dist/index.html

Spec: 2026-10-25_add-blog-i18n
Co-authored-by: GLM-5.3

src/layouts/BaseLayout.astro

@@ -12,6 +12,7 @@ import Header from '../components/Header.astro';
 import Footer from '../components/Footer.astro';
 import ThemeScript from '../components/ThemeScript.astro';
 import Keyboard from '../components/geek/Keyboard.astro';
+import { localeOf, localePath } from '../lib/locale';
 import SearchDialog from '../components/SearchDialog.astro';
 import BuildInfo from '../components/geek/BuildInfo.astro';
 import { ClientRouter } from 'astro:transitions';
@@ -23,6 +24,12 @@ interface Props {
   track?: 'reading' | 'tech';
 }
 const { title, description = 'wait4x 的博客：阅读 · 技术 · 摄影', track = 'tech' } = Astro.props;
+
+// hreflang 双向：zh 无前缀 ↔ /en/；x-default 指向 zh（中文是源语言）
+const rawPath = Astro.url.pathname.replace(/\/index\.html$/, '');
+const zhPath = localeOf(rawPath) === 'en' ? localePath(rawPath, 'zh') : rawPath;
+const enPath = localePath(rawPath, 'en');
+const withSlash = (p: string) => (p.endsWith('/') ? p : `${p}/`);
 ---
 
 <!doctype html>
@@ -37,6 +44,9 @@ const { title, description = 'wait4x 的博客：阅读 · 技术 · 摄影', tr
     <title>{title}</title>
     <ThemeScript />
     <ClientRouter />
+    <link rel="alternate" hreflang="zh" href={new URL(withSlash(zhPath), Astro.site)} />
+    <link rel="alternate" hreflang="en" href={new URL(withSlash(enPath), Astro.site)} />
+    <link rel="alternate" hreflang="x-default" href={new URL(withSlash(zhPath), Astro.site)} />
   </head>
   <body>
     <a class="skip-link" href="#main">跳到正文</a>