datalog: dl-walk handles circular substitutions without infinite loop (257/257)

Bug: dl-walk would infinite-loop on a circular substitution
(e.g. A→B and B→A simultaneously). The walk endlessly chased
the cycle. This couldn't be produced through dl-unify (which has
cycle-safe behavior via existing bindings), but raw dl-bind calls
or external manipulation of the subst dict could create it.

Fix: dl-walk now threads a visited-names list through the
recursion. If a variable name is already in the list, the walk
stops and returns the current term unchanged. Normal chained
walks are unaffected (A→B→C→42 still resolves to 42).

1 new unify test verifies circular substitutions don't hang.

lib/datalog/scoreboard.json

@@ -1,12 +1,12 @@
 {
   "lang": "datalog",
-  "total_passed": 256,
+  "total_passed": 257,
   "total_failed": 0,
-  "total": 256,
+  "total": 257,
   "suites": [
     {"name":"tokenize","passed":30,"failed":0,"total":30},
     {"name":"parse","passed":22,"failed":0,"total":22},
-    {"name":"unify","passed":28,"failed":0,"total":28},
+    {"name":"unify","passed":29,"failed":0,"total":29},
     {"name":"eval","passed":38,"failed":0,"total":38},
     {"name":"builtins","passed":23,"failed":0,"total":23},
     {"name":"semi_naive","passed":8,"failed":0,"total":8},
@@ -16,5 +16,5 @@
     {"name":"magic","passed":36,"failed":0,"total":36},
     {"name":"demo","passed":21,"failed":0,"total":21}
   ],
-  "generated": "2026-05-10T21:16:53+00:00"
+  "generated": "2026-05-11T07:20:07+00:00"
 }