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/scoreboard.json

@@ -1,8 +1,8 @@
 {
   "lang": "datalog",
-  "total_passed": 260,
+  "total_passed": 262,
   "total_failed": 0,
-  "total": 260,
+  "total": 262,
   "suites": [
     {"name":"tokenize","passed":30,"failed":0,"total":30},
     {"name":"parse","passed":22,"failed":0,"total":22},
@@ -12,9 +12,9 @@
     {"name":"semi_naive","passed":8,"failed":0,"total":8},
     {"name":"negation","passed":10,"failed":0,"total":10},
     {"name":"aggregates","passed":22,"failed":0,"total":22},
-    {"name":"api","passed":20,"failed":0,"total":20},
+    {"name":"api","passed":22,"failed":0,"total":22},
     {"name":"magic","passed":36,"failed":0,"total":36},
     {"name":"demo","passed":21,"failed":0,"total":21}
   ],
-  "generated": "2026-05-11T07:40:56+00:00"
+  "generated": "2026-05-11T07:50:41+00:00"
 }