How do I convert a sitemap-index to a single CSV?

A sitemap-index is an XML file that references multiple child sitemaps instead of listing URLs directly. To convert it to a single CSV, fetch the index, then fetch every child sitemap, then merge all URLs into one deduplicated list. The xmlsitemapmaker.com converter does this automatically — paste the sitemap-index URL and it fans out, fetches each child in parallel, dedupes, and emits one CSV with an extra Source Sitemap column so you can trace each URL back to the file it came from. Excel's built-in XML importer can't do this — it stops at the index and just shows the child sitemap URLs, not the millions of URLs inside them.