Extract _mark_backstop_done() (overflow eviction + marker write) and
_backstop_delete_doc_type() (per-type scroll + enqueue) so
_enqueue_deletes_for_disabled_types drops from cognitive complexity 17 to well
under the 15 threshold. Behavior unchanged; tests still pass.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>