Build and test on Windows (MSYS2 CLANG64)

[AmyJeanes]

Fork-internal draft, stacked on the windows branch (commaai#61). Raised here so the follow-up's own diff is visible on its own; it will be recreated against commaai/rednose once commaai#61 has merged and the comma-deps Windows wheels are on PyPI (the dependencies PR), because uv sync needs comma-deps-eigen for win_amd64. The Windows CI entry on this PR is red for that reason; the same commit is green on all three platforms from a branch that points uv at a fork-built eigen wheel: https://github.com/AmyJeanes/rednose/actions/runs/34135696496

The standalone build and ./test.sh now run from an MSYS2 CLANG64 shell with a native Python, and CI gains a windows-latest entry running the same script:

• SConstruct: the mingw tool with clang on Windows (the default tool picks MSVC), the lib prefix on the filter DLLs that ekf_load expects, libc++ linked statically so they load outside that shell, and the Cython module linked against the interpreter's import library and named .pyd.
• The filter generators run through the interpreter on Windows: cmd.exe cannot start a script by its shebang.
• The Python-side loader opens .dll filters, the counterpart of os: add support for windows commaai/rednose#61 for the cffi path.
• tests.yml: only the Windows entry names its shell; the others keep bash -e {0}, GitHub's implicit default.

🤖 Generated with Claude Code

https://claude.ai/code/session_016AgqYZYWLpwE2T5vS3nVEt

[AmyJeanes]

Superseded by a draft from windows-ci-dev, which points uv at the fork's eigen wheel so the Windows entry can run green.