s_mpi_allreduce_min/max leave var_glb undefined in non-MPI builds #1966

Summary

In builds without MPI, s_mpi_allreduce_min and s_mpi_allreduce_max (src/common/m_mpi_common.fpp) leave their intent(out) result var_glb undefined. The MPI_ALLREDUCE call is wrapped in #ifdef MFC_MPI and has no #else var_glb = var_loc. Callers on a single-process build then read garbage.

s_mpi_allreduce_sum had the same gap; #1963 adds the #else branch for it. s_mpi_allreduce_min_vec and s_mpi_reduce_int_sum already have the fallback.

Fix

Add the same branch to s_mpi_allreduce_min and s_mpi_allreduce_max:

#ifdef MFC_MPI
        ...
        call MPI_ALLREDUCE(var_loc, var_glb, 1, mpi_p, MPI_MIN, MPI_COMM_WORLD, ierr)
#else
        var_glb = var_loc
#endif

Then check whether any other scalar reduction helper is missing the same branch.

Found with Claude Code (a cleanup report that used s_mpi_allreduce_sum never printed in --no-mpi test builds).