datalog: dl-set-strategy! validates known strategy values

`dl-set-strategy!` accepted any keyword silently — typos like
`:semi_naive` or `:semiNaive` were stored uninspected and the
saturator then used the default. The user never learned their
setting was wrong.

Validator added: strategy must be one of `:semi-naive`, `:naive`,
`:magic` (the values currently recognised by the saturator and
magic-sets driver). Unknown values raise with a clear message that
lists the accepted set.

1 regression test; conformance 276/276.

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

lib/datalog/db.sx

@@ -48,17 +48,37 @@
       (and (has-key? edb rel-key)
            (has-key? (get edb rel-key) tk)))))
 
-;; 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.
+;; Evaluation strategy. Default :semi-naive (used by dl-saturate!).
+;; :naive selects dl-saturate-naive! (slower but easier to reason
+;; about). :magic is a marker — goal-directed magic-sets evaluation
+;; is invoked separately via `dl-magic-query`; setting :magic here
+;; is purely informational. Any other value is rejected so typos
+;; don't silently fall back to the default.
+(define
+  dl-strategy-values
+  (list :semi-naive :naive :magic))
+
 (define
   dl-set-strategy!
   (fn
     (db strategy)
-    (do
-      (dict-set! db :strategy strategy)
-      db)))
+    (cond
+      ((not (dl-keyword-member? strategy dl-strategy-values))
+        (error (str "dl-set-strategy!: unknown strategy " strategy
+                    " — must be one of " dl-strategy-values)))
+      (else
+        (do
+          (dict-set! db :strategy strategy)
+          db)))))
+
+(define
+  dl-keyword-member?
+  (fn
+    (k xs)
+    (cond
+      ((= (len xs) 0) false)
+      ((= k (first xs)) true)
+      (else (dl-keyword-member? k (rest xs))))))
 
 (define
   dl-get-strategy