datalog: dl-magic-query driver (204/204)

End-to-end magic-sets entry point. Given (db, query-goal):
  - copies the caller's EDB facts (relations not headed by any
    rule) into a fresh internal db
  - adds the magic seed fact
  - adds the rewritten rules
  - saturates and runs the query
  - returns the substitution list

Caller's db is untouched. Equivalent to dl-query for any
fully-stratifiable program; intended as a perf alternative on
goal-shaped queries against large recursive relations.

2 new tests: equivalence to dl-query on chain-3 ancestor, and
non-mutation of the caller's db (rules count unchanged).

lib/datalog/scoreboard.json

@@ -1,8 +1,8 @@
 {
   "lang": "datalog",
-  "total_passed": 202,
+  "total_passed": 204,
   "total_failed": 0,
-  "total": 202,
+  "total": 204,
   "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":18,"failed":0,"total":18},
+    {"name":"magic","passed":20,"failed":0,"total":20},
     {"name":"demo","passed":18,"failed":0,"total":18}
   ],
-  "generated": "2026-05-08T09:58:16+00:00"
+  "generated": "2026-05-08T10:00:29+00:00"
 }