datalog: magic-sets benefit test on disjoint-cluster graph (205/205)

Two disjoint chains, query rooted in cluster 1. Semi-naive
derives the full closure over both clusters (6 ancestor tuples).
Magic-sets only seeds magic_ancestor^bf for cluster 1, so only
2 query-relevant tuples are returned (a→b, a→c). The test
asserts both numbers, demonstrating the actual perf-shape
benefit of goal-directed evaluation.

lib/datalog/scoreboard.json

@@ -1,8 +1,8 @@
 {
   "lang": "datalog",
-  "total_passed": 204,
+  "total_passed": 205,
   "total_failed": 0,
-  "total": 204,
+  "total": 205,
   "suites": [
     {"name":"tokenize","passed":26,"failed":0,"total":26},
     {"name":"parse","passed":18,"failed":0,"total":18},
@@ -13,8 +13,8 @@
     {"name":"negation","passed":10,"failed":0,"total":10},
     {"name":"aggregates","passed":18,"failed":0,"total":18},
     {"name":"api","passed":14,"failed":0,"total":14},
-    {"name":"magic","passed":20,"failed":0,"total":20},
+    {"name":"magic","passed":21,"failed":0,"total":21},
     {"name":"demo","passed":18,"failed":0,"total":18}
   ],
-  "generated": "2026-05-08T10:00:29+00:00"
+  "generated": "2026-05-08T10:02:51+00:00"
 }