Cloudflare challenge handler.
Detects Cloudflare interstitials in fetched HTML and, when constructed
with a CF-bypass harvester (e.g. [px-camoufox::CamoufoxPool]), re-harvests
the URL to recover cf_clearance / __cf_bm plus any downstream PX cookies
that the same fetch happens to set.
See ADR-0015 (handler stub) and ADR-0020 (Camoufox path).