New bssn_em_rhs_c.C computes EM field RHS (E,B,Kpsi,Kphi) and stress-energy
tensor, then calls the C BSSN RHS kernel with source terms. Replaces empart.f90
when USE_CXX_EM_KERNEL=1. Supports all ghost_width orders via existing derivative
kernels. Controlled by USE_CXX_EM_KERNEL switch (default 0, experimental).
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
