datalog: dl-magic-query handles mixed EDB+IDB relations (225/225)

Bug: dl-magic-query was skipping EDB facts for relations that had
rules ("rule-headed"). When a single relation has both EDB facts
and rules deriving more (mixed EDB+IDB), the rewritten run would
miss the EDB portion entirely, producing too few or zero results.

Fix: copy ALL existing facts to the internal mdb regardless of
whether the relation has rules. EDB-only relations bring their
tuples; mixed relations bring both EDB and any pre-saturated IDB
(which the rewritten rules would re-derive anyway).

1 new test: link relation seeded with 3 EDB tuples plus a
recursive rule via via/2. dl-magic-query rooted at `a` returns
2 results (a→b direct, a→c via via(a,e), link(e,c)).

lib/datalog/magic.sx

@@ -397,15 +397,18 @@
                 (mdb (dl-make-db))
                 (rule-heads (dl-magic-rule-heads rules)))
           (do
-            ;; Copy EDB facts (relations not headed by any caller rule).
+            ;; Copy ALL existing facts. EDB-only relations bring their
+            ;; tuples; mixed EDB+IDB relations bring both their EDB
+            ;; portion and any pre-saturated IDB tuples (which the
+            ;; rewritten rules would re-derive anyway). Skipping facts
+            ;; for rule-headed relations would leave the magic run
+            ;; without the EDB portion of mixed relations.
             (for-each
               (fn
                 (rel)
-                (when
-                  (not (dl-member-string? rel rule-heads))
-                  (for-each
-                    (fn (t) (dl-add-fact! mdb t))
-                    (dl-rel-tuples db rel))))
+                (for-each
+                  (fn (t) (dl-add-fact! mdb t))
+                  (dl-rel-tuples db rel)))
               (keys (get db :facts)))
             ;; Seed + rewritten rules.
             (dl-add-fact! mdb (get rewritten :seed))