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/fdderivs_sh_c.C

@@ -5,7 +5,7 @@
  * fdderivs_sh — second derivatives on shell patch in (rho, sigma, R) coords.
  * Same stencil coefficients as Cartesian fdderivs. Uses symmetry_stbd.
  */
-void fdderivs_sh(const int ex[3],
+extern "C" void fdderivs_sh_(const int ex[3],
                  const double *f,
                  double *fxx, double *fxy, double *fxz,
                  double *fyy, double *fyz, double *fzz,

AMSS_NCKU_source/fdderivs_shc_c.C

@@ -2,30 +2,22 @@
 #include "share_func.h"
 #include <cstddef>
 
-/* Forward declarations */
+/* Forward declarations — Fortran-mangled names from shell C kernels */
-void fderivs_sh(const int ex[3], const double *f,
+extern "C" {
-                double *fx, double *fy, double *fz,
+void fderivs_sh_(const int ex[3], const double *f,
-                const double *X, const double *Y, const double *Z,
+                 double *fx, double *fy, double *fz,
-                double SYM1, double SYM2, double SYM3,
-                int Symmetry, int onoff, int sst);
-
-void fdderivs_sh(const int ex[3], const double *f,
-                 double *fxx, double *fxy, double *fxz,
-                 double *fyy, double *fyz, double *fzz,
                  const double *X, const double *Y, const double *Z,
                  double SYM1, double SYM2, double SYM3,
                  int Symmetry, int onoff, int sst);
 
-/*
+void fdderivs_sh_(const int ex[3], const double *f,
- * fdderivs_shc — shell second derivatives converted to Cartesian via chain rule.
+                  double *fxx, double *fxy, double *fxz,
- *
+                  double *fyy, double *fyz, double *fzz,
- * Calls fderivs_sh and fdderivs_sh internally, then applies:
+                  const double *X, const double *Y, const double *Z,
- *   f_{,ij} = sum_{a,b} (dx^a/dx^i)(dx^b/dx^j) f_{,ab} + sum_a (d^2 x^a/dx^i dx^j) f_{,a}
+                  double SYM1, double SYM2, double SYM3,
- * where a,b ∈ {rho, sigma, R}.
+                  int Symmetry, int onoff, int sst);
- */
-extern "C" {
 
-void f_fdderivs_shc(int *ex,
+void fdderivs_shc_(int *ex,
                     double *f,
                     double *fxx, double *fxy, double *fxz,
                     double *fyy, double *fyz, double *fzz,
@@ -60,8 +52,8 @@ void f_fdderivs_shc(int *ex,
         return;
     }
 
-    fderivs_sh(ex3, f, gx, gy, gz, crho, sigma, R, SYM1, SYM2, SYM3, Symmetry, Lev, sst);
+    fderivs_sh_(ex3, f, gx, gy, gz, crho, sigma, R, SYM1, SYM2, SYM3, Symmetry, Lev, sst);
-    fdderivs_sh(ex3, f, gxx, gxy, gxz, gyy, gyz, gzz, crho, sigma, R, SYM1, SYM2, SYM3, Symmetry, Lev, sst);
+    fdderivs_sh_(ex3, f, gxx, gxy, gxz, gyy, gyz, gzz, crho, sigma, R, SYM1, SYM2, SYM3, Symmetry, Lev, sst);
 
     for (size_t i = 0; i < n; ++i) {
         const double rx=drhodx[i], ry=drhody[i], rz=drhodz[i];

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,

AMSS_NCKU_source/fderivs_shc_c.C

@@ -11,16 +11,15 @@
  *   fz = drhodz * df/drho + dsigmadz * df/dsigma + dRdz * df/dR
  */
 
-// Forward declaration (defined in fderivs_sh_c.C)
+// Forward declaration (defined in fderivs_sh_c.C with extern "C" name fderivs_sh_)
-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,
-                double SYM1, double SYM2, double SYM3,
-                int Symmetry, int onoff, int sst);
-
 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,
+                 double SYM1, double SYM2, double SYM3,
+                 int Symmetry, int onoff, int sst);
 
-void f_fderivs_shc(int *ex,
+void fderivs_shc_(int *ex,
                    double *f,
                    double *fx, double *fy, double *fz,
                    double *crho, double *sigma, double *R,
@@ -40,7 +39,7 @@ void f_fderivs_shc(int *ex,
     if (!gx || !gy || !gz) { free(gx); free(gy); free(gz); return; }
 
     // Compute shell-coordinate derivatives
-    fderivs_sh(ex3, f, gx, gy, gz, crho, sigma, R, SYM1, SYM2, SYM3, Symmetry, Lev, sst);
+    fderivs_sh_(ex3, f, gx, gy, gz, crho, sigma, R, SYM1, SYM2, SYM3, Symmetry, Lev, sst);
 
     // Chain rule to Cartesian
     for (size_t i = 0; i < n; ++i) {

AMSS_NCKU_source/kodiss_sh_c.C

@@ -5,7 +5,7 @@
  * kodis_sh — Kreiss-Oliger dissipation on shell patches.
  * Same stencil coefficients as Cartesian kodis. Uses symmetry_stbd.
  */
-void kodis_sh(const int ex[3],
+extern "C" void kodis_sh_(const int ex[3],
               const double *X, const double *Y, const double *Z,
               const double *f, double *f_rhs,
               const double SoAi[2],