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.
New lib/datalog/builtins.sx: (< <= > >= = !=) and (is X expr) with
+ - * /. dl-eval-arith recursively evaluates nested compounds.
Safety analysis now walks body left-to-right tracking the bound
set: comparisons require all args bound, is RHS vars must be bound
(LHS becomes bound), = special-cases the var/non-var combos.
db.sx keeps the simple safety check as a forward-reference
fallback; builtins.sx redefines dl-rule-check-safety to the
comprehensive version. eval.sx dispatches built-ins through
dl-eval-builtin instead of erroring. 19 new tests.
