PR2: Add Windows pytest smoke CI

[audunarn]

Summary

• Add a GitHub Actions workflow for pull requests and pushes to modern_dev.
• Run on windows-latest with Python 3.13.
• Install development dependencies, install the package editable, smoke-import anystruct, and run python -m pytest.

Notes

• This intentionally starts with a single Windows job because the current application and tests have Windows-specific assumptions.
• No production, GUI, or calculation code is changed.

Verification

• Reviewed the branch compare diff.
• Not run locally in this Codex environment because the workspace here is not a checked-out repository and git is not available on PATH.
• CI should run on this draft PR and provide the real verification signal.