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/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"
 }