fluid_rho broadcast uses MPI_LOGICAL instead of mpi_p (real type)

[sbryngelson]

Bug

In `src/pre_process/m_mpi_proxy.fpp`, `fluid_rho` is a real (logical-valued) array but is broadcast with `MPI_LOGICAL` instead of `mpi_p` (the real MPI type). This causes a type mismatch in the MPI broadcast.

File: `src/pre_process/m_mpi_proxy.fpp`, line 61

Impact: On non-root MPI ranks, `fluid_rho` receives bit-reinterpreted data. Since MPI_LOGICAL and mpi_p may have different sizes, this can corrupt memory or produce incorrect values.

Severity: Critical — type mismatch in MPI broadcast corrupts data on non-root ranks.

[sbryngelson]

Fix: #1174