Fix WP_MOK to use stp instead of wp for main field data I/O

MPI-IO displacement (WP_MOK) must match the storage precision of
the data being read/written.  Main field variables are written with
mpi_io_p (which tracks stp), not mpi_p (which tracks wp).  In
mixed-precision mode stp = half (2 bytes) while wp = double (8 bytes),
so using storage_size(0._wp) gives a 4× wrong displacement.

Change all main-data WP_MOK assignments from storage_size(0._wp) to
storage_size(0._stp) across all three I/O paths:
  - src/simulation/m_data_output.fpp (write, two call sites)
  - src/simulation/m_start_up.fpp    (restart read, two call sites)
  - src/post_process/m_data_input.f90 (post-process read, two call sites)

IB marker I/O (MPI_INTEGER, 4 bytes) was already fixed to use
storage_size(0)/8 in the previous commit and is not changed here.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: sbryngelson

src/post_process/m_data_input.f90

@@ -133,7 +133,7 @@ impure subroutine s_setup_mpi_io_params(data_size, m_MOK, n_MOK, p_MOK, WP_MOK,
         m_MOK = int(m_glb + 1, MPI_OFFSET_KIND)
         n_MOK = int(n_glb + 1, MPI_OFFSET_KIND)
         p_MOK = int(p_glb + 1, MPI_OFFSET_KIND)
-        WP_MOK = int(storage_size(0._wp)/8, MPI_OFFSET_KIND)
+        WP_MOK = int(storage_size(0._stp)/8, MPI_OFFSET_KIND)
         MOK = int(1._wp, MPI_OFFSET_KIND)
         str_MOK = int(name_len, MPI_OFFSET_KIND)
         NVARS_MOK = int(sys_size, MPI_OFFSET_KIND)
@@ -521,7 +521,7 @@ impure subroutine s_read_parallel_conservative_data(t_step, m_MOK, n_MOK, p_MOK,
                 m_MOK = int(m_glb + 1, MPI_OFFSET_KIND)
                 n_MOK = int(n_glb + 1, MPI_OFFSET_KIND)
                 p_MOK = int(p_glb + 1, MPI_OFFSET_KIND)
-                WP_MOK = int(storage_size(0._wp)/8, MPI_OFFSET_KIND)
+                WP_MOK = int(storage_size(0._stp)/8, MPI_OFFSET_KIND)
                 MOK = int(1._wp, MPI_OFFSET_KIND)
                 str_MOK = int(name_len, MPI_OFFSET_KIND)
                 NVARS_MOK = int(sys_size, MPI_OFFSET_KIND)

src/simulation/m_data_output.fpp

@@ -908,7 +908,7 @@ contains
             m_MOK = int(m_glb_save + 1, MPI_OFFSET_KIND)
             n_MOK = int(n_glb_save + 1, MPI_OFFSET_KIND)
             p_MOK = int(p_glb_save + 1, MPI_OFFSET_KIND)
-            WP_MOK = int(storage_size(0._wp)/8, MPI_OFFSET_KIND)
+            WP_MOK = int(storage_size(0._stp)/8, MPI_OFFSET_KIND)
             MOK = int(1._wp, MPI_OFFSET_KIND)
             str_MOK = int(name_len, MPI_OFFSET_KIND)
             NVARS_MOK = int(sys_size, MPI_OFFSET_KIND)
@@ -976,7 +976,7 @@ contains
             m_MOK = int(m_glb + 1, MPI_OFFSET_KIND)
             n_MOK = int(n_glb + 1, MPI_OFFSET_KIND)
             p_MOK = int(p_glb + 1, MPI_OFFSET_KIND)
-            WP_MOK = int(storage_size(0._wp)/8, MPI_OFFSET_KIND)
+            WP_MOK = int(storage_size(0._stp)/8, MPI_OFFSET_KIND)
             MOK = int(1._wp, MPI_OFFSET_KIND)
             str_MOK = int(name_len, MPI_OFFSET_KIND)
             NVARS_MOK = int(alt_sys, MPI_OFFSET_KIND)

src/simulation/m_start_up.fpp

@@ -575,7 +575,7 @@ contains
                 m_MOK = int(m_glb_read + 1, MPI_OFFSET_KIND)
                 n_MOK = int(m_glb_read + 1, MPI_OFFSET_KIND)
                 p_MOK = int(m_glb_read + 1, MPI_OFFSET_KIND)
-                WP_MOK = int(storage_size(0._wp)/8, MPI_OFFSET_KIND)
+                WP_MOK = int(storage_size(0._stp)/8, MPI_OFFSET_KIND)
                 MOK = int(1._wp, MPI_OFFSET_KIND)
                 str_MOK = int(name_len, MPI_OFFSET_KIND)
                 NVARS_MOK = int(sys_size, MPI_OFFSET_KIND)
@@ -652,7 +652,7 @@ contains
                 m_MOK = int(m_glb + 1, MPI_OFFSET_KIND)
                 n_MOK = int(n_glb + 1, MPI_OFFSET_KIND)
                 p_MOK = int(p_glb + 1, MPI_OFFSET_KIND)
-                WP_MOK = int(storage_size(0._wp)/8, MPI_OFFSET_KIND)
+                WP_MOK = int(storage_size(0._stp)/8, MPI_OFFSET_KIND)
                 MOK = int(1._wp, MPI_OFFSET_KIND)
                 str_MOK = int(name_len, MPI_OFFSET_KIND)
                 NVARS_MOK = int(sys_size, MPI_OFFSET_KIND)