.github/workflows/ci-sage.yml: New

[mkoeppe]

Portability tests for macOS, Linux, Windows (mingw)

Preview: https://github.com/passagemath/upstream-m4ri/actions/runs/19993649023

[mkoeppe]

Repro of #38 in https://github.com/malb/m4ri/actions/runs/19993671749/job/57338574216?pr=40

[malb]

I'm afraid I'm not keen on merging this because it would commit us to supporting these platforms without access to them or experience with them. I don't think I could live up to that commitment.

[mkoeppe]

This can be mitigated by documenting that this CI is non-normative.

[malb]

I was, of course, wrong. We already have Windows in our CI and our README already states:

We support M4RI on Linux. Support for Windows (MSYS2/MinGW) and OS X (GCC) is provided on a best effort basis and may disappear any time.

I've merged another PR which adds OSX to our CI (given that Windows already is there, this makes sense).

I think that covers us on platforms and CI. Thanks for the PR, I really do appreciate your efforts to make the whole ecosystem portable!

[mkoeppe]

No worries, I'll just keep the CI on a branch ci-sage at https://github.com/passagemath/upstream-m4ri for next time.

[malb]

Thank you <3