HTML: Rewrite heading id anchors

[markoub]

Summary

• Map same-document HTML links like href="#lexical-analysis" to the matching imported heading text before Markdown conversion.
• Preserve non-local hash links, such as book.html#lexical-analysis and external URL fragments.
• Add focused tests for the heading anchor rewrite helper.

Fixes #217

Validation

• pnpm exec tsx --test tests/html/heading-links.test.ts
• pnpm run test
• pnpm run build
• git diff --check
• pnpm exec eslint src/formats/html.ts src/formats/html/heading-links.ts tests/html/heading-links.test.ts