Fix fluid_rho broadcast using MPI_LOGICAL instead of mpi_p

fluid_rho is a real(wp) array but is broadcast with MPI_LOGICAL type,
silently corrupting reference densities via bit reinterpretation on
non-root ranks.

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

Author: sbryngelson

src/pre_process/m_mpi_proxy.fpp

@@ -62,7 +62,7 @@ contains
             & 'igr', 'down_sample', 'simplex_perturb','fft_wrt', 'hyper_cleaning' ]
             call MPI_BCAST(${VAR}$, 1, MPI_LOGICAL, 0, MPI_COMM_WORLD, ierr)
         #:endfor
-        call MPI_BCAST(fluid_rho(1), num_fluids_max, MPI_LOGICAL, 0, MPI_COMM_WORLD, ierr)
+        call MPI_BCAST(fluid_rho(1), num_fluids_max, mpi_p, 0, MPI_COMM_WORLD, ierr)
 
         #:for VAR in [ 'x_domain%beg', 'x_domain%end', 'y_domain%beg',         &
             & 'y_domain%end', 'z_domain%beg', 'z_domain%end', 'a_x', 'a_y',    &