Enable Z4C + Shell-Patch GPU coexistence (Phase 3)

Remove the compile-time #error that blocked USE_CUDA_Z4C + WithShell.
Add GPU-to-CPU state sync at the start of both Z4C Step functions
(non-CPBC and CPBC) so shell CPU consumers read valid field data
after Cartesian GPU RHS with resident state.

Move bssn_cuda_use_resident_sync and bssn_cuda_download_level_state
_if_present from anonymous namespace to file scope in bssn_class.C
so derived classes (Z4C) can call them. Declare both in
bssn_rhs_cuda.h. Include bssn_rhs_cuda.h in Z4c_class.C.

Z4C shell RHS remains on CPU (Fortran Z4c_rhs_ss.f90) pending
future GPU kernel implementation.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

AMSS_NCKU_source/bssn_rhs_cuda.h

@@ -404,6 +404,10 @@ void bssn_cuda_release_step_ctx(void *block_tag);
 
 #ifdef __cplusplus
 }
+// C++-only helpers declared for derived equation classes (Z4C, etc.)
+// Defined in bssn_class.C. Requires MyList, Patch, var from including TU.
+bool bssn_cuda_use_resident_sync(int lev);
+void bssn_cuda_download_level_state_if_present(MyList<Patch> *PatL, MyList<var> *vars, int myrank);
 #endif
 
 #endif