blosc2: add build-blosc2.yml for riscv64 wheels

[luhenry]

• Package: blosc2
• Version: 4.11.0
• Source: https://github.com/Blosc/python-blosc2
• Docs: https://github.com/Blosc/python-blosc2

Compiles four Cython/C extensions on top of C-Blosc2 and miniexpr (fetched via CMake FetchContent, built with scikit-build-core). Upstream publishes no riscv64 wheel.

Mirrors the build_wheels job of upstream's cibuildwheels.yml.

Differs from upstream

• No cp315t wheel - numpy has no riscv64 wheel yet for cp315/cp315t anywhere.
• CIBW_TEST_ENVIRONMENT: PYTHON_GIL=1 - the extensions declare no Py_mod_gil slot, so cp314t needs it to import cleanly.

Matrix: one cp311-abi3 wheel (retested on cp312-cp314) plus cp314t, matching upstream's shape minus cp315t (see above).

Testing

• same subset upstream runs against its own sdist and abi3 wheel: test_open.py, test_vlmeta.py, ndarray/test_evaluate.py

License: OK - wheel ships LICENSE.txt plus miniexpr's bundled LICENSE/LICENSE-LIBTCC/LICENSE-SLEEF/LICENSE-TINYEXPR/THIRD_PARTY_NOTICES.md per its own CMake install rule (miniexpr bundles libtcc, LGPL-2.1, as a fetched static build, no dynamic system linkage).

Built and tested on riscv64 (cp311/cp312/cp313/cp314 abi3 + cp314t); 74 passed each.