Shell C functions must export Fortran-compatible symbols with trailing
underscore so bssn_rhs_ss.f90 and getnp4.f90 can link when WithShell is
active and USE_CXX_SHELL_KERNELS=1 replaces Fortran diff_new_sh.o.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
New files provide C equivalents of Fortran diff_new_sh.f90 and kodiss_sh.f90:
- fderivs_sh_c.C: first derivatives in shell (rho, sigma, R) coords
- fdderivs_sh_c.C: second derivatives in shell coords
- fderivs_shc_c.C: shell first derivs + chain rule to Cartesian
- fdderivs_shc_c.C: shell second derivs + chain rule to Cartesian
- kodiss_sh_c.C: Kreiss-Oliger dissipation on shell patches
Also add symmetry_stbd() C implementation and shell fh indexing to share_func.h.
Controlled by USE_CXX_SHELL_KERNELS switch (default 0, experimental).
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
