New docs/crawler.md covering the crawl path we actually run: BFS
same-host discovery, robots/feed/sitemap handling, polite vs --fast,
the shared-session + no-HEAD + crawl-delay-fix speedups, and the
content-addressed payoff. Leads on the two store-derived diagnostics:
duplicate detection (group by document_root — body, not URI) and
partial-overlap (shared chunk leaves), plus orphan finding
(sitemap − BFS-reached) and the planned crawl-report webmaster tools.
Honest pros/cons: orphans invisible to crawl by design, single-host,
no JS execution, --fast is anti-social off your own turf.
