Native DFT-D4 dispersion interface for OpenQP.
Thin bind(C) shim over the dftd4 Fortran library (statically linked into
liboqp). Replaces the former dftd4 Python (cffi) package, which is capped
at Python <= 3.12. This routine has no Python dependency and is called
directly through the existing cffi boundary (see include/oqp.h).