datalog: tag co-occurrence demo (169/169)

Adds (cotagged P T1 T2) — P has both T1 and T2 with T1 != T2 — and
(tag-pair-count T1 T2 N) which counts posts cotagged with each
distinct (T1, T2) pair. Demonstrates count aggregation against a
recursive-then-aggregated stream of derived tuples.

2 new demo tests: cooking + vegetarian co-occurrence on a small
data set, and a count-of-co-occurrences query.

lib/datalog/demo.sx

@@ -84,6 +84,26 @@
        (authored Author P)
        (tagged P cooking)))))
 
+;; ── Demo 5: tag co-occurrence ───────────────────────────────────
+;; "Posts tagged with both T1 AND T2." Useful for narrowed-down
+;; recommendations like "vegetarian cooking" posts.
+;;
+;; EDB:
+;;   (tagged POST TAG)
+;; IDB:
+;;   (cotagged POST T1 T2)        — post has both T1 and T2 (T1 != T2)
+;;   (popular-pair T1 T2 N)       — count of posts cotagged (T1, T2)
+(define
+  dl-demo-tag-cooccur-rules
+  (quote
+    ((cotagged P T1 T2 <- (tagged P T1) (tagged P T2) (!= T1 T2))
+     ;; Distinct (T1, T2) pairs that occur somewhere.
+     (tag-pair T1 T2 <- (cotagged P T1 T2))
+     (tag-pair-count T1 T2 N
+       <-
+       (tag-pair T1 T2)
+       (count N P (cotagged P T1 T2))))))
+
 ;; ── Loader stub ──────────────────────────────────────────────────
 ;; Wiring to PostgreSQL would replace these helpers with calls into
 ;; rose-ash's internal HTTP RPC (fetch_data → /internal/data/...).