fix(seo): align JSON-LD with Yoast/RankMath conventions

- Wrap category pages in a self-contained @graph (WebSite + CollectionPage)
- Set canonical @id on CollectionPage to its URL (no hash fragment)
- Expand isPartOf to typed object {"@type": "WebSite", "@id": ...}
- Extract _website_node() and ISPARTOF_WEBSITE constants to avoid repetition
- Update tests to assert @graph structure on category pages

Co-Authored-By: Claude <noreply@anthropic.com>