fix(mdbx): restore optional build

[LimiNode]

Summary

• add public Logger backend accessors used by typed logger macros and the MDBX example
• add mdbx-containers to_bytes/from_bytes support for MdbxLogger value types
• add explicit C++17 guard and MDBX CI job

Replacement PR

This replaces #113. The previous PR was opened from feat/mdbx-logger after main already contained the squashed MDBX feature work, so GitHub showed the old MDBX diff again and reported merge conflicts. This branch was recreated from current origin/main and contains only the incremental fix.

Verification

• cmake -S . -B build-mdbx-clean -G "MinGW Makefiles" -DLOGIT_CPP_BUILD_TESTS=ON -DLOGIT_CPP_BUILD_EXAMPLES=ON -DLOGIT_WITH_MDBX=ON -DLOGIT_USE_SUBMODULES=ON -DCMAKE_CXX_STANDARD=17
• cmake --build build-mdbx-clean
• ctest --test-dir build-mdbx-clean --output-on-failure -R mdbx_logger_test
• cmake -S . -B build-default-clean -G "MinGW Makefiles" -DLOGIT_CPP_BUILD_TESTS=ON -DCMAKE_CXX_STANDARD=17
• cmake --build build-default-clean
• ctest --test-dir build-default-clean --output-on-failure --quiet
• git diff --check origin/main...HEAD

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