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/conformance.conf

@@ -8,6 +8,7 @@ PRELOADS=(
   lib/datalog/parser.sx
   lib/datalog/unify.sx
   lib/datalog/db.sx
+  lib/datalog/builtins.sx
   lib/datalog/eval.sx
 )
 
@@ -16,4 +17,5 @@ SUITES=(
   "parse:lib/datalog/tests/parse.sx:(dl-parse-tests-run!)"
   "unify:lib/datalog/tests/unify.sx:(dl-unify-tests-run!)"
   "eval:lib/datalog/tests/eval.sx:(dl-eval-tests-run!)"
+  "builtins:lib/datalog/tests/builtins.sx:(dl-builtins-tests-run!)"
 )