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.md

@@ -1,12 +1,12 @@
 # datalog scoreboard
 
-**256 / 256 passing** (0 failure(s)).
+**257 / 257 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
 | tokenize | 30 | 30 | ok |
 | parse | 22 | 22 | ok |
-| unify | 28 | 28 | ok |
+| unify | 29 | 29 | ok |
 | eval | 38 | 38 | ok |
 | builtins | 23 | 23 | ok |
 | semi_naive | 8 | 8 | ok |