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/builtins.sx

@@ -262,6 +262,19 @@
                       missing
                       " — they must be bound by an earlier "
                       "positive body literal")))))))
+        (define
+          dl-process-agg!
+          (fn
+            (lit)
+            (let
+              ((result-var (nth lit 1)))
+              ;; Aggregate goal vars are existentially quantified within
+              ;; the aggregate; nothing required from outer context. The
+              ;; result var becomes bound after the aggregate fires.
+              (when
+                (dl-var? result-var)
+                (dl-add-bound! (list (symbol->string result-var)))))))
+
         (define
           dl-process-lit!
           (fn
@@ -271,6 +284,7 @@
               (cond
                 ((and (dict? lit) (has-key? lit :neg))
                   (dl-process-neg! lit))
+                ((dl-aggregate? lit) (dl-process-agg! lit))
                 ((dl-eq? lit) (dl-process-eq! lit))
                 ((dl-is? lit) (dl-process-is! lit))
                 ((dl-comparison? lit) (dl-process-cmp! lit))