feat: add fundamental theorem of topos theory eval problem#397
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Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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This PR adds the fundamental theorem of topos theory (§54 of the Knill survey): the slice category
E/Xof an elementary toposEis again an elementary topos. The trusted helperIsTopos(a non-hole) bundles the four Mathlib classes that make up "elementary topos". Mathlib has finite limits and cartesian-monoidal structure onOver Xand a subobject-classifier class, but neitherMonoidalClosed (Over X)(the locally-cartesian-closed upgrade) norHasSubobjectClassifier (Over X).🤖 Prepared with Claude Code