XML sitemap checker

The scanner reads tags, not a full XML parser: it finds urlset and sitemapindex roots, extracts loc entries, and flags missing, blank, relative, and unclosed blocks. It does not validate XML schema, namespaces, lastmod dates, or changefreq values. A sitemap index is followed one level down, up to 10 children. The proxy hands back at most the first 4,096 characters of each response body, so larger sitemaps are checked on that prefix and the report says so. The URL list stops at 500 entries; the true count sits beside it.