v4: Further fixes for f2py, macos, and spack

[mathomp4]

Fixed

• Fixed f2py and f2py3 detection in Spack environments where the f2py
  executable lives in the py-numpy package's bin/ directory rather than
  alongside the Python interpreter. FindF2PY.cmake and FindF2PY3.cmake now
  ask Python directly (import numpy.f2py) to locate the correct f2py
  binary. A two-pass search is used: the preferred locations (Python bin/ and
  numpy-derived bin/) are checked first with NO_DEFAULT_PATH, then a normal
  PATH search is used as a fallback to preserve behaviour on non-Spack
  (e.g. module-based) systems.
• Fixed find_package(Python3) picking up a different Python 3 interpreter
  (e.g. a newer Homebrew Python) instead of the one already found by
  find_package(Python) in Spack environments. esma_python.cmake now pins
  Python3_EXECUTABLE to Python_EXECUTABLE when find_package(Python) has
  already resolved a Python 3 interpreter, ensuring both find the same Python.

Changed

• Added Python_FIND_VIRTUALENV FIRST and Python3_FIND_VIRTUALENV FIRST to
  esma_python.cmake so that an active Spack python-venv on PATH is
  preferred over system or framework Pythons.