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