`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>