M0-M11 狩夜人笔记初始实现：11里程碑+86测试

tests/unit/roundtrip.test.ts

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+import { describe, it, expect } from 'vitest';
+import { promises as fs } from 'node:fs';
+import nodePath from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { unified } from 'unified';
+import remarkParse from 'remark-parse';
+import remarkStringify from 'remark-stringify';
+import remarkGfm from 'remark-gfm';
+import remarkMath from 'remark-math';
+
+const __dirname = nodePath.dirname(fileURLToPath(import.meta.url));
+const fxDir = nodePath.join(__dirname, '..', 'fixtures', 'roundtrip');
+
+const parse = (md: string) => unified().use(remarkParse).use(remarkGfm).use(remarkMath).parse(md);
+const roundtrip = (md: string) =>
+  unified().use(remarkParse).use(remarkGfm).use(remarkMath)
+    .use(remarkStringify).use(remarkGfm).use(remarkMath).processSync(md).toString();
+
+function normalize(node: any): any {
+  if (node === null || typeof node !== 'object') return node;
+  if (Array.isArray(node)) return node.map(normalize);
+  const { position, ...rest } = node;
+  for (const k of Object.keys(rest)) rest[k] = normalize(rest[k]);
+  return rest;
+}
+
+const CASES = ['emphasis-nested','escaping','tables','codeblock','tasklist','headings','thematic-break','blockquote','lists-nested','whitespace','breaks','links','images','math'];
+
+describe('Markdown 往返 fixture 集（mdast 等价）', () => {
+  it('fixture 数量足够', async () => {
+    const names = (await fs.readdir(fxDir)).filter(f => f.endsWith('.md') && f !== 'README.md');
+    expect(names.length).toBeGreaterThanOrEqual(15);
+  });
+
+  for (const name of CASES) {
+    it(name + ' 往返 mdast 等价', async () => {
+      const md = await fs.readFile(nodePath.join(fxDir, name + '.md'), 'utf8');
+      const mdPrime = roundtrip(md);
+      expect(normalize(parse(mdPrime))).toEqual(normalize(parse(md)));
+    });
+  }
+});