Turn any sitemap.xml into clean JSON: URL entries with loc, lastmod and priority, plus sitemap-index detection with the child sitemap list. Handles gzipped .gz sitemaps (magic-byte detection, zip-bomb guarded) and namespaced XML; pass follow=1 to also expand up to 3 child sitemaps of an index in one call. SSRF-checked fetches, 10 MB per document, up to 50,000 URLs with a truncated flag.