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/scoreboard.md

@@ -1,6 +1,6 @@
 # datalog scoreboard
 
-**181 / 181 passing** (0 failure(s)).
+**183 / 183 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
@@ -11,6 +11,6 @@
 | builtins | 19 | 19 | ok |
 | semi_naive | 8 | 8 | ok |
 | negation | 10 | 10 | ok |
-| aggregates | 16 | 16 | ok |
+| aggregates | 18 | 18 | ok |
 | api | 13 | 13 | ok |
 | demo | 18 | 18 | ok |