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>

plans/datalog-on-sx.md

@@ -15,7 +15,7 @@ for rose-ash data (e.g. federation graph, content relationships).
 
 ## Status (rolling)
 
-`bash lib/datalog/conformance.sh` → **275/275 across 11 suites**
+`bash lib/datalog/conformance.sh` → **276/276 across 11 suites**
 (tokenize, parse, unify, eval, builtins, semi_naive, negation, aggregates,
 api, magic, demo). Source is ~3100 LOC, tests ~2900 LOC, public API
 documented in `lib/datalog/datalog.sx`.
@@ -320,6 +320,13 @@ large graphs.
 
 _Newest first._
 
+- 2026-05-11 — `dl-set-strategy!` accepted arbitrary keyword values
+  silently. Typos like `:semi_naive` or `:semiNaive` were stored
+  uninspected; the saturator then used the default and the user
+  never learned their setting was a typo. Validator added: strategy
+  must be one of `:semi-naive`, `:naive`, `:magic`. 1 regression test;
+  276/276.
+
 - 2026-05-11 — Anonymous-variable renamer collided with user-written
   `_anon<N>` symbols. The renamer started counter at 0 and produced
   `_anon1, _anon2, ...` unconditionally; if the user wrote