datalog: dl-saturated? fixpoint predicate (212/212)

Returns true iff one more saturation step would derive no new
tuples. Walks every rule under the current bindings and short-
circuits as soon as one derivation would add a fresh tuple.
Useful in tests that want to assert "no work left" after a call,
or for tooling that wants to know whether `dl-saturate!` would
do anything.

3 new eval tests cover the after-saturation, before-saturation,
and after-assert states.

lib/datalog/scoreboard.json

@@ -1,13 +1,13 @@
 {
   "lang": "datalog",
-  "total_passed": 209,
+  "total_passed": 212,
   "total_failed": 0,
-  "total": 209,
+  "total": 212,
   "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":"eval","passed":25,"failed":0,"total":25},
+    {"name":"eval","passed":28,"failed":0,"total":28},
     {"name":"builtins","passed":19,"failed":0,"total":19},
     {"name":"semi_naive","passed":8,"failed":0,"total":8},
     {"name":"negation","passed":10,"failed":0,"total":10},
@@ -16,5 +16,5 @@
     {"name":"magic","passed":22,"failed":0,"total":22},
     {"name":"demo","passed":18,"failed":0,"total":18}
   ],
-  "generated": "2026-05-08T10:13:00+00:00"
+  "generated": "2026-05-08T10:15:09+00:00"
 }