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