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/tests/eval.sx

@@ -359,6 +359,16 @@
           (do (dl-set-strategy! db :magic) (dl-get-strategy db)))
         :magic)
 
+      ;; Unknown strategy values are rejected so typos don't silently
+      ;; fall back to the default.
+      (dl-et-test!
+        "unknown strategy rejected"
+        (dl-et-throws?
+          (fn ()
+            (let ((db (dl-make-db)))
+              (dl-set-strategy! db :semi_naive))))
+        true)
+
       ;; dl-saturated?: no-work-left predicate.
       (dl-et-test! "saturated? after saturation"
         (let ((db (dl-program