datalog: aggregates work as top-level query goals (183/183)

Bug: dl-match-lit (the naive matcher used by dl-find-bindings)
was missing dl-aggregate? dispatch — it was only present in
dl-fbs-aux (semi-naive). Symptom:
  (dl-query db '(count N X (p X)))
silently returned ().

Two fixes:
- Add aggregate branch to dl-match-lit before the positive case.
- dl-query-user-vars now projects only the result var (first arg)
  of an aggregate goal — the aggregated var and inner-goal vars
  are existentials and should not leak into substitutions.

2 new aggregate tests cover count and findall as direct query goals.

lib/datalog/eval.sx

@@ -83,6 +83,7 @@
     (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)))
@@ -426,16 +427,35 @@
         (for-each
           (fn
             (g)
-            (let
-              ((tgt
-                 (if (and (dict? g) (has-key? g :neg)) (get g :neg) g)))
-              (for-each
-                (fn
-                  (v)
+            (cond
+              ((and (dict? g) (has-key? g :neg))
+                (for-each
+                  (fn
+                    (v)
+                    (when
+                      (and (not (= v "_")) (not (dl-member-string? v seen)))
+                      (append! seen v)))
+                  (dl-vars-of (get g :neg))))
+              ((dl-aggregate? g)
+                ;; Only the result var (first arg of the aggregate
+                ;; literal) is user-facing. The aggregated var and
+                ;; any vars in the inner goal are internal.
+                (let ((r (nth g 1)))
                   (when
-                    (and (not (= v "_")) (not (dl-member-string? v seen)))
-                    (append! seen v)))
-                (dl-vars-of tgt))))
+                    (dl-var? r)
+                    (let ((rn (symbol->string r)))
+                      (when
+                        (and (not (= rn "_"))
+                             (not (dl-member-string? rn seen)))
+                        (append! seen rn))))))
+              (else
+                (for-each
+                  (fn
+                    (v)
+                    (when
+                      (and (not (= v "_")) (not (dl-member-string? v seen)))
+                      (append! seen v)))
+                  (dl-vars-of g)))))
           goals)
         seen))))