Add C implementations of shell-patch derivative kernels (WithShell support)

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>

AMSS_NCKU_source/makefile.inc

@@ -51,6 +51,12 @@ USE_CXX_Z4C_KERNELS ?= 1
 ## Note: this requires USE_CXX_KERNELS=1 because the wrapper reuses the C BSSN kernel.
 USE_CXX_ESCALAR_KERNEL ?= 1
 
+## BSSN-EM RHS switch
+##   1 : use BSSN-EM C kernel (bssn_em_rhs_c.C) on the normal patch path
+##   0 : keep the original Fortran empart.f90 RHS for the EM fields (default)
+## Note: experimental, requires USE_CXX_KERNELS=1
+USE_CXX_EM_KERNEL ?= 0
+
 ## Cached transfer switch
 ##   auto (default): enable for BSSN vacuum, keep other paths on the safe uncached path
 ##   1             : force cached Sync/Restrict/OutBd transfer on evolution hot paths