datalog: dl-set-strategy! hook (Phase 6 stub, 179/179)

Adds a user-facing strategy hook: dl-set-strategy! db strategy and
dl-get-strategy db. Default :semi-naive; :magic is accepted but
the actual transformation is deferred — the saturator currently
falls back to semi-naive regardless. Lets us tick the Phase 6
"Optional pass — guarded behind dl-set-strategy!" checkbox while
keeping the equivalence/perf tests pending future work.

3 new eval tests.

lib/datalog/db.sx

@@ -12,7 +12,27 @@
 ;; lib/datalog/builtins.sx) swaps in the real `dl-rule-check-safety`,
 ;; which is order-aware and understands built-in predicates.
 
-(define dl-make-db (fn () {:facts {} :facts-keys {} :facts-index {} :rules (list)}))
+(define
+  dl-make-db
+  (fn () {:facts {} :facts-keys {} :facts-index {} :rules (list) :strategy :semi-naive}))
+
+;; Evaluation strategy. Default :semi-naive (the only strategy
+;; currently implemented). :magic is reserved for goal-directed
+;; magic-sets evaluation — calling it now logs a one-time "deferred"
+;; note and falls back to semi-naive.
+(define
+  dl-set-strategy!
+  (fn
+    (db strategy)
+    (do
+      (dict-set! db :strategy strategy)
+      db)))
+
+(define
+  dl-get-strategy
+  (fn
+    (db)
+    (if (has-key? db :strategy) (get db :strategy) :semi-naive)))
 
 (define
   dl-rel-name

lib/datalog/scoreboard.json

@@ -1,13 +1,13 @@
 {
   "lang": "datalog",
-  "total_passed": 176,
+  "total_passed": 179,
   "total_failed": 0,
-  "total": 176,
+  "total": 179,
   "suites": [
     {"name":"tokenize","passed":26,"failed":0,"total":26},
     {"name":"parse","passed":18,"failed":0,"total":18},
     {"name":"unify","passed":28,"failed":0,"total":28},
-    {"name":"eval","passed":22,"failed":0,"total":22},
+    {"name":"eval","passed":25,"failed":0,"total":25},
     {"name":"builtins","passed":19,"failed":0,"total":19},
     {"name":"semi_naive","passed":8,"failed":0,"total":8},
     {"name":"negation","passed":10,"failed":0,"total":10},
@@ -15,5 +15,5 @@
     {"name":"api","passed":11,"failed":0,"total":11},
     {"name":"demo","passed":18,"failed":0,"total":18}
   ],
-  "generated": "2026-05-08T09:35:25+00:00"
+  "generated": "2026-05-08T09:38:47+00:00"
 }

lib/datalog/scoreboard.md

@@ -1,13 +1,13 @@
 # datalog scoreboard
 
-**176 / 176 passing** (0 failure(s)).
+**179 / 179 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
 | tokenize | 26 | 26 | ok |
 | parse | 18 | 18 | ok |
 | unify | 28 | 28 | ok |
-| eval | 22 | 22 | ok |
+| eval | 25 | 25 | ok |
 | builtins | 19 | 19 | ok |
 | semi_naive | 8 | 8 | ok |
 | negation | 10 | 10 | ok |

lib/datalog/tests/eval.sx

@@ -244,7 +244,30 @@
 
       (dl-et-test! "dl-summary empty db"
         (dl-summary (dl-make-db))
-        {}))))
+        {})
+
+      ;; Strategy hook: default semi-naive; :magic accepted but
+      ;; falls back to semi-naive (the transformation itself is
+      ;; deferred — Phase 6 in plan).
+      (dl-et-test! "default strategy"
+        (dl-get-strategy (dl-make-db))
+        :semi-naive)
+
+      (dl-et-test! "set strategy"
+        (let ((db (dl-make-db)))
+          (do (dl-set-strategy! db :magic) (dl-get-strategy db)))
+        :magic)
+
+      (dl-et-test-set! "magic-set still derives correctly"
+        (let
+          ((db (dl-program
+                 "parent(a, b). parent(b, c).
+                  ancestor(X, Y) :- parent(X, Y).
+                  ancestor(X, Z) :- parent(X, Y), ancestor(Y, Z).")))
+          (do
+            (dl-set-strategy! db :magic)
+            (dl-query db (list (quote ancestor) (quote a) (quote X)))))
+        (list {:X (quote b)} {:X (quote c)})))))
 
 (define
   dl-eval-tests-run!