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/tests/demo.sx

@@ -179,6 +179,41 @@
           (quote (can-access X blog)))
         (list {:X (quote carol)}))
 
+      ;; ── Cooking posts (canonical Phase 10 example) ─────────
+      (dl-demo-test-set! "cooking posts by network"
+        (dl-query
+          (dl-demo-make
+            (quote
+              ((follows me alice) (follows alice bob) (follows alice carol)
+               (authored alice p1) (authored bob p2)
+               (authored carol p3) (authored carol p4)
+               (tagged p1 travel) (tagged p2 cooking)
+               (tagged p3 cooking) (tagged p4 books)))
+            dl-demo-cooking-rules)
+          (quote (cooking-post-by-network me P)))
+        (list {:P (quote p2)} {:P (quote p3)}))
+
+      (dl-demo-test-set! "cooking — direct follow only"
+        (dl-query
+          (dl-demo-make
+            (quote
+              ((follows me bob)
+               (authored bob p1) (authored bob p2)
+               (tagged p1 cooking) (tagged p2 books)))
+            dl-demo-cooking-rules)
+          (quote (cooking-post-by-network me P)))
+        (list {:P (quote p1)}))
+
+      (dl-demo-test-set! "cooking — none in network"
+        (dl-query
+          (dl-demo-make
+            (quote
+              ((follows me bob)
+               (authored bob p1) (tagged p1 books)))
+            dl-demo-cooking-rules)
+          (quote (cooking-post-by-network me P)))
+        (list))
+
       (dl-demo-test-set! "no access without grant"
         (dl-query
           (dl-demo-make