[richard-gledhill]

What

Adds a new cookiecutter option, mypy_check_linux_and_windows (n by default).

When type_checker is mypy and this is set to y, make check in the generated project runs mypy once per platform:

uv run mypy --platform linux
uv run mypy --platform win32

With the default n, behaviour is unchanged: a single uv run mypy. The option has no effect when type_checker is ty.

Why

mypy checks code against a single platform, so branches guarded by sys.platform == "win32" (or "linux") are skipped on the other OS. Running mypy for both platforms catches type errors in platform-specific code, which matters for projects that support Linux and Windows.

This is an opt-in option for a fairly narrow case: projects with platform-specific code paths that need to type-check on both Linux and Windows. It is off by default, so projects that don't need it see no change, and it increases mypy's runtime when enabled.

Changes

• cookiecutter.json: new mypy_check_linux_and_windows option, ordered ["n", "y"] so the default is unchanged.
• Makefile template: the check target now renders one of three variants (mypy, mypy on both platforms, ty).
• tests/test_combinations.py: new test_makefile_type_checker and a mypy-type-checker-multi-y combination. The test asserts on whole Makefile lines.
• docs/features/linting.md: new "Checking Linux and Windows" section under mypy.

Notes

• The option is prompted for even when ty is chosen, because cookiecutter cannot skip a prompt based on an earlier answer. If you would rather avoid that, I can fold it into type_checker instead (for example mypy, mypy-multi-platform, ty).
• I ran make check, make test and make docs locally. I did not run tox because the other Python versions are not installed here, so I am relying on CI for that.