datalog: magic-vs-semi work-shape test on chain-12 (209/209)

Demonstrates the practical effect of goal-directed evaluation:
chain of 12 nodes, semi-naive derives the full ancestor closure
(78 = 12·13/2 tuples), while a magic-rooted query at node 0
returns only its 12 descendants. Concrete check that magic
limits derivation to the query's transitive cone.

lib/datalog/tests/magic.sx

@@ -227,6 +227,35 @@
       ;; Magic-sets benefit: query touches only one cluster of a
       ;; multi-component graph. Semi-naive derives the full closure
       ;; over both clusters; magic only the seeded one.
+      ;; Magic-vs-semi work shape: chain of 12. Semi-naive
+      ;; derives the full closure (78 = 12·13/2). A magic query
+      ;; rooted at node 0 returns the 12 descendants only —
+      ;; demonstrating that magic limits derivation to the
+      ;; query's transitive cone.
+      (dl-mt-test! "magic vs semi work-shape on chain-12"
+        (let
+          ((source (str
+                     "parent(0, 1). parent(1, 2). parent(2, 3). "
+                     "parent(3, 4). parent(4, 5). parent(5, 6). "
+                     "parent(6, 7). parent(7, 8). parent(8, 9). "
+                     "parent(9, 10). parent(10, 11). parent(11, 12). "
+                     "ancestor(X, Y) :- parent(X, Y). "
+                     "ancestor(X, Z) :- parent(X, Y), ancestor(Y, Z).")))
+          (let
+            ((db1 (dl-make-db)) (db2 (dl-make-db)))
+            (do
+              (dl-load-program! db1 source)
+              (dl-saturate! db1)
+              (dl-load-program! db2 source)
+              (let
+                ((semi-count (len (dl-relation db1 "ancestor")))
+                  (magic-count
+                    (len (dl-magic-query
+                           db2 (list (quote ancestor) 0 (quote X))))))
+                ;; Magic returns only descendants of 0 (12 of them).
+                (and (= semi-count 78) (= magic-count 12))))))
+        true)
+
       (dl-mt-test! "magic skips irrelevant clusters"
         (let
           ;; Two disjoint chains. Query is rooted in cluster 1.