Index a citation with MeSH headings

Reinforce NLM MeSH indexing order from medical-library training. Headings are not assigned from the title alone; the abstract and check tags come first.

1. Read the abstract and the methods, not only the title.
2. Select the most specific MeSH terms available.
3. Add required check tags (human, age, etc.).
4. Apply subheadings only where the heading allows them.
5. Review against the current MeSH browser.
6. Do not keyword-stuff synonyms that are already entry terms.