Multi-sitemap discovery

Flowpane searches multiple sources:

1. robots.txt — parse all Sitemap: directives
2. Standard locations — /sitemap.xml and /sitemap_index.xml
3. Platform-specific — based on detected platform
4. Index expansion — child sitemaps fetched and analysed

Deduplication by URL (first discovery wins). Safety: 5s timeout per fetch, max 50 children per index.