Remove MPI dependency, replace with single-process stub for non-MPI builds

- Add mpi_stub.h providing all MPI types/constants/functions as no-ops
  (nprocs=1, myrank=0) with memcpy-based Allreduce and clock_gettime Wtime
- Replace #include <mpi.h> with conditional #ifdef MPI_STUB in 31 files
  (19 headers + 12 source files) preserving ability to build with real MPI
- Change makefile.inc: CLINKER mpiicpx->icpx, add -DMPI_STUB to CXXAPPFLAGS
- Update makefile_and_run.py: run ./ABE directly instead of mpirun -np N
- Set MPI_processes=1 in AMSS_NCKU_Input.py

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

AMSS_NCKU_source/Newton.C

@@ -8,7 +8,11 @@
 #include <limits.h>
 #include <float.h>
 #include <math.h>
+#ifdef MPI_STUB
+#include "mpi_stub.h"
+#else
 #include <mpi.h>
+#endif
 
 #include "util_Table.h"
 #include "cctk.h"