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/scoreboard.json

@@ -1,13 +1,13 @@
 {
   "lang": "datalog",
-  "total_passed": 275,
+  "total_passed": 276,
   "total_failed": 0,
-  "total": 275,
+  "total": 276,
   "suites": [
     {"name":"tokenize","passed":31,"failed":0,"total":31},
     {"name":"parse","passed":23,"failed":0,"total":23},
     {"name":"unify","passed":29,"failed":0,"total":29},
-    {"name":"eval","passed":43,"failed":0,"total":43},
+    {"name":"eval","passed":44,"failed":0,"total":44},
     {"name":"builtins","passed":26,"failed":0,"total":26},
     {"name":"semi_naive","passed":8,"failed":0,"total":8},
     {"name":"negation","passed":12,"failed":0,"total":12},
@@ -16,5 +16,5 @@
     {"name":"magic","passed":37,"failed":0,"total":37},
     {"name":"demo","passed":21,"failed":0,"total":21}
   ],
-  "generated": "2026-05-11T09:34:17+00:00"
+  "generated": "2026-05-11T09:40:12+00:00"
 }