202607121639

CI: add Windows build target using MSYS2 MinGW-w32

Add windows-latest to the build matrix. The Windows job:
- Uses msys2/setup-msys2@v2 with MINGW32 environment
- Installs mingw-w64-i686-gcc, make and git packages
- Builds the default 'pcap' make target (npcap SDK is already
  tracked in the repo under npcap/)
- The makefile auto-detects Windows from the -w32 compiler triplet,
  enables -static linking, -lws2_32, and -Lnpcap/Lib

The artifact upload path uses a glob (udp2raw-*{matrix.os}*) to
handle both extensionless binaries (Linux/macOS) and .exe (Windows).
The upload-releases job downloads all udp2raw-* artifacts and
publishes them to GitHub Releases unchanged.