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/...).

lib/datalog/scoreboard.json

@@ -1,8 +1,8 @@
 {
   "lang": "datalog",
-  "total_passed": 167,
+  "total_passed": 169,
   "total_failed": 0,
-  "total": 167,
+  "total": 169,
   "suites": [
     {"name":"tokenize","passed":26,"failed":0,"total":26},
     {"name":"parse","passed":18,"failed":0,"total":18},
@@ -13,7 +13,7 @@
     {"name":"negation","passed":10,"failed":0,"total":10},
     {"name":"aggregates","passed":16,"failed":0,"total":16},
     {"name":"api","passed":11,"failed":0,"total":11},
-    {"name":"demo","passed":13,"failed":0,"total":13}
+    {"name":"demo","passed":15,"failed":0,"total":15}
   ],
-  "generated": "2026-05-08T09:16:59+00:00"
+  "generated": "2026-05-08T09:20:09+00:00"
 }

lib/datalog/scoreboard.md

@@ -1,6 +1,6 @@
 # datalog scoreboard
 
-**167 / 167 passing** (0 failure(s)).
+**169 / 169 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
@@ -13,4 +13,4 @@
 | negation | 10 | 10 | ok |
 | aggregates | 16 | 16 | ok |
 | api | 11 | 11 | ok |
-| demo | 13 | 13 | ok |
+| demo | 15 | 15 | ok |

lib/datalog/tests/demo.sx

@@ -214,6 +214,29 @@
           (quote (cooking-post-by-network me P)))
         (list))
 
+      ;; ── Tag co-occurrence ──────────────────────────────────
+      (dl-demo-test-set! "cotagged posts"
+        (dl-query
+          (dl-demo-make
+            (quote
+              ((tagged p1 cooking) (tagged p1 vegetarian)
+               (tagged p2 cooking) (tagged p2 quick)
+               (tagged p3 vegetarian)))
+            dl-demo-tag-cooccur-rules)
+          (quote (cotagged P cooking vegetarian)))
+        (list {:P (quote p1)}))
+
+      (dl-demo-test-set! "tag pair count"
+        (dl-query
+          (dl-demo-make
+            (quote
+              ((tagged p1 cooking) (tagged p1 vegetarian)
+               (tagged p2 cooking) (tagged p2 quick)
+               (tagged p3 cooking) (tagged p3 vegetarian)))
+            dl-demo-tag-cooccur-rules)
+          (quote (tag-pair-count cooking vegetarian N)))
+        (list {:N 2}))
+
       (dl-demo-test-set! "no access without grant"
         (dl-query
           (dl-demo-make