datalog: aggregation count/sum/min/max (Phase 8, 134/134)

New lib/datalog/aggregates.sx: (count R V Goal), (sum R V Goal),
(min R V Goal), (max R V Goal). dl-eval-aggregate runs
dl-find-bindings on the goal under the outer subst, collects
distinct values of V, applies the operator, binds R. Empty input:
count/sum return 0; min/max produce no binding (rule fails).

Group-by emerges naturally from outer-subst substitution into the
goal — `popular(P) :- post(P), count(N, U, liked(U, P)), >=(N, 3).`
counts per-post.

Stratifier extended: dl-aggregate-dep-edge contributes a
negation-like edge so the aggregate's goal relation is fully
derived before the aggregate fires (non-monotonicity respected).
Safety relaxed for aggregates: goal-internal vars are existentials,
only the result var becomes bound.

lib/datalog/conformance.conf

@@ -9,6 +9,7 @@ PRELOADS=(
   lib/datalog/unify.sx
   lib/datalog/db.sx
   lib/datalog/builtins.sx
+  lib/datalog/aggregates.sx
   lib/datalog/strata.sx
   lib/datalog/eval.sx
 )
@@ -21,4 +22,5 @@ SUITES=(
   "builtins:lib/datalog/tests/builtins.sx:(dl-builtins-tests-run!)"
   "semi_naive:lib/datalog/tests/semi_naive.sx:(dl-semi-naive-tests-run!)"
   "negation:lib/datalog/tests/negation.sx:(dl-negation-tests-run!)"
+  "aggregates:lib/datalog/tests/aggregates.sx:(dl-aggregates-tests-run!)"
 )