datalog: built-ins + body arithmetic + order-aware safety (Phase 4, 106/106)

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.

lib/datalog/eval.sx

@@ -4,11 +4,11 @@
 ;; The Herbrand base is finite (no function symbols) so termination is
 ;; guaranteed by the language.
 ;;
-;; Phase 3 handles only positive EDB literals. Negation (Phase 7) and
-;; arithmetic built-ins (Phase 4) raise a clear error if encountered.
+;; Body literal kinds handled here:
+;;   positive (rel arg ... arg)  → match against EDB+IDB tuples (dl-match-positive)
+;;   built-in (< X Y), (is X e)  → constraint via dl-eval-builtin (Phase 4)
+;;   negation {:neg lit}         → Phase 7
 
-;; Match a positive literal against every tuple in the relation; return
-;; a list of extended substitutions, one per successful unification.
 (define
   dl-match-positive
   (fn
@@ -38,13 +38,13 @@
       ((and (dict? lit) (has-key? lit :neg))
         (error "datalog: negation not yet supported (Phase 7)"))
       ((dl-builtin? lit)
-        (error "datalog: built-in predicate not yet supported (Phase 4)"))
+        (let
+          ((s (dl-eval-builtin lit subst)))
+          (if (nil? s) (list) (list s))))
       ((and (list? lit) (> (len lit) 0))
         (dl-match-positive lit db subst))
       (else (error (str "datalog: unknown body-literal shape: " lit))))))
 
-;; Recursively find all substitutions satisfying a list of body literals
-;; under `subst`. Returns a list of substitutions.
 (define
   dl-find-bindings
   (fn
@@ -66,7 +66,6 @@
               options)
             results))))))
 
-;; Apply a rule once. Returns true if any new tuple was derived.
 (define
   dl-apply-rule!
   (fn
@@ -83,7 +82,6 @@
           (dl-find-bindings body db (dl-empty-subst)))
         new?))))
 
-;; Iterate rules to fixpoint.
 (define
   dl-saturate!
   (fn
@@ -106,9 +104,6 @@
         (dl-sat-loop)
         db))))
 
-;; Run a goal against the saturated db. Returns a list of substitutions
-;; (each restricted to the variables that appeared in `goal`). Duplicate
-;; projections are deduplicated so users get the set of answers.
 (define
   dl-query
   (fn
@@ -131,7 +126,6 @@
             substs)
           results)))))
 
-;; Project a subst down to a given set of variable names.
 (define
   dl-project-subst
   (fn