datalog: dl-retract! preserves EDB in mixed relations

A "mixed" relation has both user-asserted facts AND rules with the
same head. Previously dl-retract! wiped every rule-head relation
wholesale before re-saturating — the saturator only re-derives the
IDB portion, so explicit EDB facts vanished even for a no-op retract
of a non-existent tuple. Repro:

  (let ((db (dl-program "p(a). p(b). p(X) :- q(X). q(c).")))
    (dl-retract! db (quote (p z)))
    (dl-query db (quote (p X))))

went from {a, b, c} to just {c}.

Fix: track :edb-keys provenance in the db.

  - dl-make-db now allocates an :edb-keys dict.
  - dl-add-fact! (public) marks (rel-key, tuple-key) in :edb-keys.
  - New internal dl-add-derived! does the append without marking.
  - Saturator (semi-naive + naive driver) now calls dl-add-derived!.
  - dl-retract! strips only the IDB-derived portion of rule-head
    relations (anything not in :edb-keys) and preserves the EDB
    portion through the re-saturate pass.

2 new regression tests; conformance 262/262.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/datalog/eval.sx

@@ -130,7 +130,7 @@
             (s)
             (let
               ((derived (dl-apply-subst head s)))
-              (when (dl-add-fact! db derived) (set! new? true))))
+              (when (dl-add-derived! db derived) (set! new? true))))
           (dl-find-bindings body db (dl-empty-subst)))
         new?))))
 
@@ -326,7 +326,7 @@
       (fn
         (lit)
         (when
-          (dl-add-fact! db lit)
+          (dl-add-derived! db lit)
           (let
             ((rel (dl-rel-name lit)))
             (do