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/tests/aggregates.sx

@@ -260,6 +260,21 @@
           (list (quote low) (quote M)))
         (list))
 
+      ;; Aggregates as the top-level query goal (regression for
+      ;; dl-match-lit aggregate dispatch and projection cleanup).
+      (dl-at-test-set! "count as query goal"
+        (dl-query
+          (dl-program "p(1). p(2). p(3). p(4).")
+          (list (quote count) (quote N) (quote X) (list (quote p) (quote X))))
+        (list {:N 4}))
+
+      (dl-at-test-set! "findall as query goal"
+        (dl-query
+          (dl-program "p(1). p(2). p(3).")
+          (list (quote findall) (quote L) (quote X)
+                (list (quote p) (quote X))))
+        (list {:L (list 1 2 3)}))
+
       (dl-at-test-set! "distinct counted once"
         (dl-query
           (dl-program