datalog: aggregation count/sum/min/max (Phase 8, 134/134)

New lib/datalog/aggregates.sx: (count R V Goal), (sum R V Goal),
(min R V Goal), (max R V Goal). dl-eval-aggregate runs
dl-find-bindings on the goal under the outer subst, collects
distinct values of V, applies the operator, binds R. Empty input:
count/sum return 0; min/max produce no binding (rule fails).

Group-by emerges naturally from outer-subst substitution into the
goal — `popular(P) :- post(P), count(N, U, liked(U, P)), >=(N, 3).`
counts per-post.

Stratifier extended: dl-aggregate-dep-edge contributes a
negation-like edge so the aggregate's goal relation is fully
derived before the aggregate fires (non-monotonicity respected).
Safety relaxed for aggregates: goal-internal vars are existentials,
only the result var becomes bound.

lib/datalog/scoreboard.json

@@ -1,8 +1,8 @@
 {
   "lang": "datalog",
-  "total_passed": 124,
+  "total_passed": 134,
   "total_failed": 0,
-  "total": 124,
+  "total": 134,
   "suites": [
     {"name":"tokenize","passed":26,"failed":0,"total":26},
     {"name":"parse","passed":18,"failed":0,"total":18},
@@ -10,7 +10,8 @@
     {"name":"eval","passed":15,"failed":0,"total":15},
     {"name":"builtins","passed":19,"failed":0,"total":19},
     {"name":"semi_naive","passed":8,"failed":0,"total":8},
-    {"name":"negation","passed":10,"failed":0,"total":10}
+    {"name":"negation","passed":10,"failed":0,"total":10},
+    {"name":"aggregates","passed":10,"failed":0,"total":10}
   ],
-  "generated": "2026-05-08T08:20:41+00:00"
+  "generated": "2026-05-08T08:28:29+00:00"
 }