pyamg: add build-pyamg.yml for riscv64 wheels

[luhenry]

• Package: pyamg
• Version: 5.3.0
• Source: https://github.com/pyamg/pyamg
• Docs: https://github.com/pyamg/pyamg

Compiles pyamg's amg_core pybind11 C++ extensions (algebraic multigrid solver kernels) into per-interpreter wheels. Upstream publishes no riscv64 wheel.

Mirrors upstream's wheels.yml.

Differs from upstream

• Test command adds -I - the checked-out pyamg/ source tree otherwise shadows the installed wheel, and amg_core/__init__.py has no pure-Python fallback so the shadowed import fails collection outright.

Testing

• Wheel-level: upstream's own pytest, run via --pyargs pyamg against the installed wheel. Reproduces on any platform (confirmed on macOS/arm64 too), not riscv64-specific.

License: OK

Patches

• 0001-fix-util-utils-duplicate-zeros-in-filter_operator.patch - Backport [upstream 9f8299b]. Without it, filter_operator returns wrong values under scipy>=1.17 (a stale has_canonical_format flag skips deduplication of an explicit-zero entry).
• 0002-fix-util-linalg-condest-non-symmetric-matvec-overri.patch - To upstream (can't file against pyamg/pyamg from this project). Without it, condest(symmetric=False) silently ignores its matvec override under scipy>=1.17, returning roughly sqrt of the true condition number.

Built on cp312; 269 passed, 1 skipped.