Fix Windows wheel build for free-threaded Python (#219)

• Initial plan

• Fix Windows wheel build: use PYBIND11_FINDPYTHON instead of deprecated PYBIND11_NEWPYTHON

The free-threaded CPython 3.14 (cp314t) build on Windows fails with: LINK : fatal error LNK1104: cannot open file ‘python314.lib’

The root cause is that PYBIND11_NEWPYTHON is deprecated/unrecognized in pybind11 3.0.x, causing it to fall back to FindPythonLibsNew.cmake which doesn’t properly handle the ‘t’ suffix for free-threaded Python on Windows.

PYBIND11_FINDPYTHON is the correct variable that tells pybind11 to use CMake’s modern FindPython module, which correctly handles free-threaded Python library naming (python314t.lib vs python314.lib).