datalog: db + naive eval + safety analysis (Phase 3, 87/87)

db.sx: facts indexed by relation name, rules list, dl-add-fact!
(rejects non-ground), dl-add-rule! (rejects unsafe — head vars
not in positive body). eval.sx: dl-saturate! fixpoint, dl-query
with deduped projected results. Negation and arithmetic raise
clear errors (Phase 4/7 to follow). 15 eval tests: transitive
closure, sibling, same-gen, grandparent, cyclic reach, safety.

lib/datalog/conformance.conf

@@ -7,10 +7,13 @@ PRELOADS=(
   lib/datalog/tokenizer.sx
   lib/datalog/parser.sx
   lib/datalog/unify.sx
+  lib/datalog/db.sx
+  lib/datalog/eval.sx
 )
 
 SUITES=(
   "tokenize:lib/datalog/tests/tokenize.sx:(dl-tokenize-tests-run!)"
   "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!)"
 )