#53 feat: support standalone HTML documents

Summary

Closes #52.

• Add standalone HTML detection from <!doctype html> or an <html> root, including HTML exports saved with a .doc extension.
• Reuse the EPUB HTML-to-model walker for headings, formatting, links, tables, lists, CSS visibility, and external images.
• Treat HTML void elements such as unclosed <meta> and <img> as empty elements in the shared DOM parser.
• Expose html / .html / .htm through the Rust, Node, Python, and WASM APIs and update docs/tests.
• MHTML remains out of scope for this change.

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --locked
• npm test
• python -m unittest discover -s python/tests
• node --test wasm/test.mjs

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