pre_process broadcasts integer bc_*%beg/%end with the real datatype (mpi_p) #1561

Symptom: none observable today — but it is undefined behavior that has been working by accident.

Mechanism: src/pre_process/m_mpi_proxy.fpp:55-65 (master) uses one combined Fypp loop that broadcasts x/y/z_domain (reals, correct) and bc_x/y/z%beg/%end — which are int_bounds_info INTEGER members — all with mpi_p: an 8-byte real transfer over a 4-byte integer that only behaves because of struct adjacency. simulation and post_process correctly use MPI_INTEGER for the same members.

Introduced: the 2022 compile-time case-dictionary squash (c9c5d922, 2022-06-02, @henryleberre, pre-PR-workflow era) changed pre's bc_x from a real bounds_info to int_bounds_info while leaving the real-typed broadcast in place; the later combined loop merely inherited it. Tagging @henryleberre for context.

Fix: #1553 splits the loop and broadcasts the BC codes with MPI_INTEGER.