Add SEO test coverage

The only existing test file is imagetools.test.js. Nothing covers the meta/sitemap/feed surface — meaning any of the audit fixes can silently regress without notice.

Test targets

• Meta.svelte — title fallback chain, OG/Twitter parity, canonical, noindex, keywords, empty-content suppression
• sitemap-appendix.xml — included URLs match excluded/draft/noindex rules
• feed.json — JSON Feed 1.1 schema (version, home_page_url, feed_url, items)
• feed.xml — sanitizeContentForRSS regex, HTML escaping, no double slashes in <id>
• parsePublicationDate / publicationDateToISO — DD-MM-YYYY, ISO, and invalid input
• pathToPublicURL — route-group stripping, nested groups
• getReadNext — tag-overlap ranking, fallback to recent, excluded/draft filtering