Adds a windows-latest job to the release matrix: MSYS2/MinGW-w64 toolchain,
Chez 10.4.1 built from source (ta6nt), the same build-joltc flow, packaged
as a zip of joltc.exe. The whole job runs in the msys2 shell so cc/xxd/paths
behave; the produced binary is a plain Windows executable.
Platform seams: bld-nt? with the winsock/COM/registry link set, no -rdynamic
under MinGW, GetModuleFileName as the launcher's self-path on _WIN32, and
built binaries (joltc itself and jolt-build outputs) normalize to a .exe
suffix. workflow_dispatch added so the matrix can be dry-run without tagging;
the release upload step only fires on tags.
For #205.
b2aa757af2