feat: support optional complex scalar arguments

[krystophny]

Summary

Closes #371.

UnwrappablesRemover.visit_Argument dropped every optional complex scalar argument, so present(z) could never be exercised from Python. Required complex scalars were wrapped normally. This removes that restriction, after which the f2py path handles optional complex scalars directly.

For --direct-c, two further gaps are fixed so complex scalar values work:

• the scalar argument parser had no complex case; it now reads a Python complex via PyComplex_AsCComplex.
• default complex mapped to double _Complex / NPY_CDOUBLE, a different size than Fortran's single-precision default, which dropped the imaginary part. It now maps to float _Complex / NPY_COMPLEX64, mirroring default real -> float. double complex and complex(8) still map to double; complex(dp) via the kind map is unchanged.

Verification

• test/test_transform.py::TestTransform::test_optional_complex_scalar_kept asserts the optional complex argument survives the transform. On master it fails: AssertionError: 'z' not found in ['flag'].
• test/test_directc.py complex mapping tests updated to the corrected single-precision defaults (and double complex / complex(4) added); 29 tests pass.
• /tmp/f90wrap371-venv/bin/python -m pytest --import-mode=importlib -q /home/ert/code/f90wrap-dev/f90wrap/test/test_transform.py::TestTransform::test_optional_complex_scalar_kept /home/ert/code/f90wrap-dev/f90wrap/test/test_directc.py: 30 passed in 0.27s.
• examples/issue371_optional_complex round-trips a complex value through present(z) on both the f2py and --direct-c paths.

Compiler matrix for examples/issue371_optional_complex, using make -f Makefile.meson test with DIRECTC=no and DIRECTC=yes:

Compiler	Version	f2py	direct-C
gfortran	GCC 16.1.1	pass, Ran 2 tests ... OK	pass, Ran 2 tests ... OK
flang	LLVM flang 22.1.6	pass, Ran 2 tests ... OK	pass, Ran 2 tests ... OK
ifx	2025.0.4	pass, Ran 2 tests ... OK	pass, Ran 2 tests ... OK
nvfortran	26.3	pass, Ran 2 tests ... OK	pass, Ran 2 tests ... OK

The ifx runs need Intel's runtime library directory on the link path: LIBRARY_PATH=/opt/intel/oneapi/compiler/2025.0/lib.

Note

The absent-argument case for optional scalars under --direct-c (passing None so present() is .false.) is handled by a separate change for #369.