feat(web): add M0 live preview editor prototype

- CodeMirror 6 editor with Obsidian/Typora-style live preview: block
  markup revealed per line, inline markup revealed per element
- Fenced code highlighting via Shiki (github-dark, JS regex engine) in a
  Web Worker, languages loaded on demand
- Lezer syntax tree to HTML renderer for share pages, checked against
  the CommonMark 0.31.2 spec (640/652; 12 known parser deviations)
- Use markdownLanguage directly to avoid bundling lang-html/css/js

web/src/main.ts

@@ -0,0 +1,11 @@
+// M0 原型入口：只挂载编辑器，M1 替换为 Vue 外壳
+import { createEditor } from './editor'
+import sample from './demo/sample.md?raw'
+import './themes/base.css'
+
+// ?repeat=N 将示例重复 N 次，用于大文档性能测试
+const repeat = Number(new URLSearchParams(location.search).get('repeat') ?? 1)
+const doc = Array.from({ length: Math.max(1, repeat) }, () => sample).join('\n')
+
+const view = createEditor({ parent: document.getElementById('app')!, doc })
+;(window as unknown as { rawmarkView: typeof view }).rawmarkView = view