Fix shell C kernel symbol names for Fortran linkage (fderivs_sh_ etc.)

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>

AMSS_NCKU_source/fderivs_sh_c.C

@@ -10,7 +10,7 @@
  * - SoA is 2-element only (x/y), no z-symmetry
  * - sst parameter (shell surface type, not used in stencil computation)
  */
-void fderivs_sh(const int ex[3],
+extern "C" void fderivs_sh_(const int ex[3],
                 const double *f,
                 double *fx, double *fy, double *fz,
                 const double *X, const double *Y, const double *Z,