test: add roundtrip consistency tests

[luojiyin1987]

Closes #31

Changes

• Add __tests__/roundtrip.spec.ts with 10 test cases covering:
  • Basic markdown (heading, paragraph, list)
  • GFM table
  • GFM task list
  • GFM strikethrough
  • Frontmatter
  • Block math
  • Inline math
  • Container directive
  • Leaf directive
  • Mixed content

Implementation

• stripPosition helper removes position info before comparison
• assertRoundtrip verifies: parse → stringify → parse produces equivalent AST

Verification

pnpm run build && pnpm test

All 46 tests pass.