Fix all compiler warnings with strict warning flags

[maxwbuckley]

Summary

• Enable strict compiler warnings and fix ~220+ warnings across the codebase
• Add HNSWLIB_WERROR CMake option to treat warnings as errors
• Ensure all library headers, examples, and tests compile cleanly with strict flags

Changes

Library headers:

• Convert C-style casts to C++ casts (static_cast, reinterpret_cast, const_cast)
• Fix const correctness in distance functions and data pointers
• Mark unused parameters with /*name*/ syntax
• Add missing override keywords on virtual functions
• Fix shadow variable warnings
• Fix sign comparison warnings (int vs size_t)
• Add HNSWLIB_MAYBE_UNUSED macro for conditionally-used SIMD functions
• Fix member initializer order in visited_list_pool.h

Examples and tests:

• Fix sign comparison warnings in loops
• Mark unused lambda parameters
• Fix shadow variables
• Add missing override keywords

Build infrastructure:

• Update CMakeLists.txt with proper warning flags for GCC/Clang/MSVC
• Add HNSWLIB_WERROR option to treat warnings as errors

Note

The .github/workflows/build.yml change to enable -DHNSWLIB_WERROR=ON in CI could not be included in this PR due to GitHub token permissions. Maintainers may want to add this line to the build step:

cmake .. -DHNSWLIB_WERROR=ON

Test plan

• All C++ tests pass (searchKnnCloserFirst_test, epsilon_search_test, searchKnnWithFilter_test, multivector_search_test, multiThread_replace_test, multiThreadLoad_test)
• All Python binding tests pass
• Build succeeds with -DHNSWLIB_WERROR=ON (warnings as errors)

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