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/scoreboard.md

@@ -1,6 +1,6 @@
 # datalog scoreboard
 
-**224 / 224 passing** (0 failure(s)).
+**225 / 225 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
@@ -13,5 +13,5 @@
 | negation | 10 | 10 | ok |
 | aggregates | 19 | 19 | ok |
 | api | 20 | 20 | ok |
-| magic | 24 | 24 | ok |
+| magic | 25 | 25 | ok |
 | demo | 21 | 21 | ok |