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.

plans/datalog-on-sx.md

@@ -284,6 +284,11 @@ large graphs.
 
 _Newest first._
 
+- 2026-05-08 — Convenience: `dl-eval source query-source`. Parses
+  both strings, builds a db, saturates, runs the query, returns
+  the substitution list. Single-call user-friendly entry. 2 new
+  api tests cover ancestor and multi-goal queries.
+
 - 2026-05-08 — Phase 6 stub: `dl-set-strategy! db strategy` and
   `dl-get-strategy db` user-facing hooks. Default `:semi-naive`;
   `:magic` is accepted but the actual transformation is deferred,