datalog: magic regression tests from bug-hunt round (242/242)

Bug-hunt round probed magic-sets against many edge cases. No new
bugs surfaced. Added regression tests for two patterns that
exercise the worklist post-fix:

- 3-stratum program (a → c via not-b → d via not-banned).
  Distinct rule heads at three strata; magic must rewrite each.
- Aggregate-derived chain (count(src) → cnt → active threshold).
  Magic correctly handles multi-step aggregate dependencies.

Magic-sets is robust against: 3-stratum negation, aggregate
chains, mutual recursion, all-bound goals, multi-arity rules,
diagonal queries, EDB-only goals, and rules whose body has
identical positive lits.

lib/datalog/tests/magic.sx

@@ -242,6 +242,34 @@
             (= (len semi) (len magic))))
         true)
 
+      ;; 3-stratum program under magic — distinct rule heads at
+      ;; strata 0/1/2 must all rewrite via the worklist.
+      (dl-mt-test! "magic 3-stratum program"
+        (let
+          ((db (dl-program
+                 "a(1). a(2). a(3). b(2).
+                  c(X) :- a(X), not(b(X)).
+                  d(X) :- c(X), not(banned(X)).
+                  banned(3).")))
+          (let
+            ((semi (dl-query db (list (quote d) (quote X))))
+              (magic (dl-magic-query db (list (quote d) (quote X)))))
+            (= (len semi) (len magic))))
+        true)
+
+      ;; Aggregate -> derived -> threshold chain via magic.
+      (dl-mt-test! "magic aggregate-derived chain"
+        (let
+          ((db (dl-program
+                 "src(1). src(2). src(3).
+                  cnt(N) :- count(N, X, src(X)).
+                  active(N) :- cnt(N), >=(N, 2).")))
+          (let
+            ((semi (dl-query db (list (quote active) (quote N))))
+              (magic (dl-magic-query db (list (quote active) (quote N)))))
+            (= (len semi) (len magic))))
+        true)
+
       ;; Multi-relation rewrite chain: query r4 → propagate to r3,
       ;; r2, r1, a. The worklist must process all of them; an
       ;; earlier bug stopped after only the head pair.