datalog: unification + substitution + 28 tests (Phase 2, 72/72)

dl-unify returns immutable extended subst dict (or nil). dl-walk
chases bindings, dl-apply-subst recursively resolves vars. Lists
unify element-wise so arithmetic compounds work too. dl-ground? and
dl-vars-of for safety analysis (Phase 3).

lib/datalog/scoreboard.json

@@ -1,11 +1,12 @@
 {
   "lang": "datalog",
-  "total_passed": 44,
+  "total_passed": 72,
   "total_failed": 0,
-  "total": 44,
+  "total": 72,
   "suites": [
     {"name":"tokenize","passed":26,"failed":0,"total":26},
-    {"name":"parse","passed":18,"failed":0,"total":18}
+    {"name":"parse","passed":18,"failed":0,"total":18},
+    {"name":"unify","passed":28,"failed":0,"total":28}
   ],
-  "generated": "2026-05-07T23:30:07+00:00"
+  "generated": "2026-05-07T23:34:02+00:00"
 }