Add GitHub Actions CI workflow (8-build matrix)

[Copilot]

Adds CI via GitHub Actions covering GCC, Clang, Clang+libc++, and MSVC × Debug/Release.

Matrix (8 builds)

Compiler	Debug	Release	OS
GCC	✓	✓	ubuntu-latest
Clang	✓	✓	ubuntu-latest
Clang + libc++	✓	✓	ubuntu-latest
MSVC	✓	✓	windows-latest

Notable implementation details

• fail-fast: false — all 8 jobs run regardless of individual failures
• Clang + libc++: sets both CMAKE_C_COMPILER=clang and CMAKE_CXX_COMPILER=clang++; CMake applies EXE_LINKER_FLAGS to the C compiler detection test, so both must be clang or configure fails
• MSVC: uses the default Visual Studio multi-config generator — no CMAKE_BUILD_TYPE at configure time; build type passed via --config at build time, binary at build\<config>\test.exe
• Linux builds validated locally before commit