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.md

@@ -1,6 +1,6 @@
 # datalog scoreboard
 
-**179 / 179 passing** (0 failure(s)).
+**181 / 181 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
@@ -12,5 +12,5 @@
 | semi_naive | 8 | 8 | ok |
 | negation | 10 | 10 | ok |
 | aggregates | 16 | 16 | ok |
-| api | 11 | 11 | ok |
+| api | 13 | 13 | ok |
 | demo | 18 | 18 | ok |