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/api.sx

@@ -149,6 +149,26 @@
         (dl-saturate! db)
         db))))
 
+;; ── Convenience: single-call source + query ───────────────────
+;; (dl-eval source query-source) parses both, builds a db, saturates,
+;; runs the query, returns the substitution list. The query source
+;; should be `?- goal[, goal ...].` — the parser produces a clause
+;; with :query containing a list of literals which is fed straight
+;; to dl-query.
+(define
+  dl-eval
+  (fn
+    (source query-source)
+    (let
+      ((db (dl-program source))
+        (queries (dl-parse query-source)))
+      (cond
+        ((= (len queries) 0) (error "dl-eval: query string is empty"))
+        ((not (has-key? (first queries) :query))
+          (error "dl-eval: second arg must be a `?- ...` query clause"))
+        (else
+          (dl-query db (get (first queries) :query)))))))
+
 (define
   dl-rule-head-rels
   (fn