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Item → recs: co-occurrence evidence fused with same-category, broad-category-by-distance, geo-prior, and content-ANN lists.

Speculative (impression-time) fetches pass prefetch=true so their insertion slates carry context "prefetched": true — an insertion with no joined impression is then "delivered, unjudged", not a phantom serve (docs/EXPLORE_CLIENT_CACHE_DESIGN.md §0.5). session_id (the scroll session) enables the session-seen soft cap (spec §4 F4). sources is a CSV of supported pool ids: cooccur, content_text, geo_hex_near, geo_hex_metro, geo_hex_peers, geo_same_category, geo_broad_category, and geo_pop. Unknown ids are ignored. It restricts which pools run; for example, sources=content_text serves pure turbopuffer ANN, the cold-start feel.