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/eval.sx

@@ -206,6 +206,7 @@
               (cond
                 ((and (dict? lit) (has-key? lit :neg))
                   (dl-match-negation (get lit :neg) db subst))
+                ((dl-aggregate? lit) (dl-eval-aggregate lit db subst))
                 ((dl-builtin? lit)
                   (let
                     ((s (dl-eval-builtin lit subst)))