Fix Windows build: write py.typed at install time instead of copying it

[garth-wells]

Summary

Fixes a regression introduced in #1048 that currently breaks both Windows CI jobs on main.

python/basix/py.typed is not a tracked source file — it's listed in .gitignore and only ever exists as a build artifact generated by nanobind's stub tooling (on Unix, nanobind_add_stub's non-INSTALL_TIME mode writes it into the source tree as part of the normal build step, before the separate install(FILES ...) copies it into the wheel).

#1048's Windows fix assumed it was a static committed file like on Unix, and added:

install(FILES ${CMAKE_CURRENT_SOURCE_DIR}/basix/py.typed DESTINATION basix)

On a fresh checkout that source path doesn't exist, so file(INSTALL) hard-fails during cmake --install, breaking the wheel build entirely on both Windows jobs (confirmed via run 30586685876):

CMake Error at .../cmake_install.cmake:48 (file):
  file INSTALL cannot find "D:/a/basix/basix/python/basix/py.typed": File
  exists.

Fix

Replace the broken install(FILES ...) with an unconditional

install(CODE "file(WRITE \"${CMAKE_INSTALL_PREFIX}/basix/py.typed\" \"\")")

which writes the (empty, per PEP 561) marker directly at the install destination — no dependency on any pre-existing source file, and no dependency on nanobind_add_stub's INSTALL_TIME import of basix._basixcpp succeeding (that import is what can fail on the Windows split build per the existing code comment, which is the actual root cause of the original, silently-tolerated "missing py.typed marker" mypy note this was trying to fix).

Test plan

• Verified standalone with a minimal CMake project that this exact install(CODE ...) construct creates the file correctly, both when the destination directory pre-exists and when it doesn't (auto-created).
• gersemi --diff clean.
• Windows CI (this PR) — both jobs should build successfully again.

🤖 Generated with Claude Code