Add macOS matrix job (arm64/macos-15, x86-64/macos-13) using Homebrew.
Add Windows ARM64 job using windows-11-arm runner with MSYS2 CLANGARM64.
Fix memory_monitor.cpp: add __APPLE__ branch using sysctl hw.memsize/hw.usermem
instead of Linux-only sysinfo/proc.
Guard X11 display crash handler with __linux__, add Windows GlobalMemoryStatusEx
path in memory_monitor, guard warden cache paths with APPDATA on Windows, and
make pkg-config optional in CMakeLists with a find_library fallback. Add Windows
x86-64 CI job using MSYS2 MINGW64 to the build workflow.