xmhuffman: add build-xmhuffman.yml for riscv64 wheels

[luhenry]

• Package: xmhuffman
• Version: 0.3.0
• Source: https://github.com/Hugoberry/xmhuffman-cython
• Docs: https://github.com/Hugoberry/xmhuffman-cython

Compiles a Cython extension (xmhuffman.pyx + src/xmhuffman_kernel.c) implementing Microsoft xVelocity/Vertipaq canonical-Huffman string decoding. Upstream publishes no riscv64 wheel.

Mirrors upstream's build.yml, building directly from the checkout rather than a self-built sdist: upstream's own sdist ships the cythonized .c but not the .pyx, and setup.py unconditionally re-cythonizes, so a wheel can't be built from that sdist at all (confirmed locally against both the PyPI sdist and one freshly built from the tag).

Differs from upstream

• Nothing beyond the riscv64 image.

Testing

• same as upstream: e2e_test.py, the smoke test upstream's own test_wheels job runs against the installed wheel.

License: OK

Built and smoke-tested locally on cp312 (native wheel from the checkout); e2e_test.py passed.