datalog: stratifier rejects recursion through aggregation (165/165)

Bug: dl-check-stratifiable iterated body literals looking only for
explicit :neg literals, missing aggregate cycles. Now also walks
aggregates via dl-aggregate-dep-edge — q(N) :- count(N, X, q(X))
correctly errors out at saturation time.

3 new tests cover:
- recursion-through-aggregation rejected
- negation + aggregation coexist when in different strata
- min over empty derived relation produces no result

lib/datalog/scoreboard.md

@@ -1,6 +1,6 @@
 # datalog scoreboard
 
-**162 / 162 passing** (0 failure(s)).
+**165 / 165 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 | 13 | 13 | ok |
+| aggregates | 16 | 16 | ok |
 | api | 9 | 9 | ok |
 | demo | 13 | 13 | ok |