datalog: cooking-posts canonical demo (Phase 10, 162/162)

Adds the canonical Phase 10 example from the plan: "Posts about
cooking by people I follow (transitively)." dl-demo-cooking-rules
defines reach over the follow graph (recursive transitive closure)
and cooking-post-by-network joining reach + authored + (tagged P
cooking). 3 new demo tests cover transitive network, direct-only
follow, and empty-network cases.

lib/datalog/demo.sx

@@ -64,6 +64,26 @@
      (subgroup-trans X Z <- (subgroup X Y) (subgroup-trans Y Z))
      (can-access A R <- (in-group A G) (allowed G R)))))
 
+;; ── Demo 4: cooking-posts (the canonical Phase 10 query) ────────
+;; "Posts about cooking by people I follow (transitively)."
+;; Combines federation (follows + transitive reach), authoring,
+;; tagging — the rose-ash multi-domain join.
+;;
+;; EDB:
+;;   (follows ACTOR-A ACTOR-B)
+;;   (authored ACTOR POST)
+;;   (tagged POST TAG)
+(define
+  dl-demo-cooking-rules
+  (quote
+    ((reach Me Them <- (follows Me Them))
+     (reach Me Them <- (follows Me X) (reach X Them))
+     (cooking-post-by-network Me P
+       <-
+       (reach Me Author)
+       (authored Author P)
+       (tagged P cooking)))))
+
 ;; ── Loader stub ──────────────────────────────────────────────────
 ;; Wiring to PostgreSQL would replace these helpers with calls into
 ;; rose-ash's internal HTTP RPC (fetch_data → /internal/data/...).