datalog: dl-eval source + query convenience (181/181)

Single-call entry: dl-eval source-string query-string parses
both, builds a db via dl-program, saturates implicitly, runs
the query (extracted from the parsed `?- ...` clause), and
returns the substitution list.

Most user-friendly path:
  (dl-eval "parent(a, b). ..." "?- ancestor(a, X).")

2 new api tests cover ancestor and multi-goal usage.

lib/datalog/scoreboard.json

@@ -1,8 +1,8 @@
 {
   "lang": "datalog",
-  "total_passed": 179,
+  "total_passed": 181,
   "total_failed": 0,
-  "total": 179,
+  "total": 181,
   "suites": [
     {"name":"tokenize","passed":26,"failed":0,"total":26},
     {"name":"parse","passed":18,"failed":0,"total":18},
@@ -12,8 +12,8 @@
     {"name":"semi_naive","passed":8,"failed":0,"total":8},
     {"name":"negation","passed":10,"failed":0,"total":10},
     {"name":"aggregates","passed":16,"failed":0,"total":16},
-    {"name":"api","passed":11,"failed":0,"total":11},
+    {"name":"api","passed":13,"failed":0,"total":13},
     {"name":"demo","passed":18,"failed":0,"total":18}
   ],
-  "generated": "2026-05-08T09:38:47+00:00"
+  "generated": "2026-05-08T09:40:51+00:00"
 }