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/scoreboard.json

@@ -1,12 +1,13 @@
 {
   "lang": "datalog",
-  "total_passed": 72,
+  "total_passed": 87,
   "total_failed": 0,
-  "total": 72,
+  "total": 87,
   "suites": [
     {"name":"tokenize","passed":26,"failed":0,"total":26},
     {"name":"parse","passed":18,"failed":0,"total":18},
-    {"name":"unify","passed":28,"failed":0,"total":28}
+    {"name":"unify","passed":28,"failed":0,"total":28},
+    {"name":"eval","passed":15,"failed":0,"total":15}
   ],
-  "generated": "2026-05-07T23:34:02+00:00"
+  "generated": "2026-05-07T23:40:51+00:00"
 }