Add a GitHub Actions CI workflow with a full build matrix covering
linux, macOS, and Windows with both GCC and Clang compilers, Debug
and Release build types, and optional ASan+UBSan sanitizers (24 builds
total). Windows builds use MSYS2 for GCC/Clang toolchains. The workflow
relies on CMake and CTest to drive configuration, building, and testing.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
