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/tests/unify.sx

@@ -129,6 +129,15 @@
             ((s2 (dl-unify (quote Y) (quote tom) s1)))
             (dl-walk (quote X) s2)))
         (quote tom))
+
+      ;; Walk with circular substitution must not infinite-loop.
+      ;; Cycles return the current term unchanged.
+      (dl-ut-test!
+        "walk circular subst no hang"
+        (let ((s (dl-bind (quote B) (quote A)
+                   (dl-bind (quote A) (quote B) (dl-empty-subst)))))
+          (dl-walk (quote A) s))
+        (quote A))
       (dl-ut-test!
         "apply subst on tuple"
         (let