Set up test infrastructure #8

Set up external test suites and a harness to run them.

Tasks:

• Add html5lib-tests as git submodule under tests/html5lib-tests/
• Add Test262 as git submodule under tests/test262/
• Create test harness (integration test or script) that:
  • Reads html5lib tokenizer test JSON files
  • Runs each test case against our (initially empty) HTML tokenizer
  • Reports pass/fail/skip counts
• Similar skeleton harness for Test262 against our (initially empty) JS engine
• Document in CLAUDE.md how to run these test suites

The harnesses should work with empty implementations (reporting 0 pass, N fail/skip). This gives us a continuous measure of progress as we implement features.

No dependencies on other Phase 1 issues.