Fix undefined allreduce min/max/sum results in non-MPI builds

[sbryngelson]

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Acknowledgement

• I confirm this PR meets the above expectations and reflects my own understanding and real-world context.

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Fixes #1966

Change

In non-MPI builds, s_mpi_allreduce_min and s_mpi_allreduce_max in src/common/m_mpi_common.fpp left their intent(out) var_glb undefined. Added #else var_glb = var_loc.

• s_mpi_allreduce_sum had the same gap. It is also fixed in the unmerged Keep species mass fractions in [0, 1] by construction instead of clipping #1963; repeated here so this PR stands alone (trivial overlap, same one-line branch).
• Audit of the rest of the file found one more: s_mpi_gather_data left its allocatable gathered_vector unallocated without MPI. Added #else gathered_vector = my_vector (single process: the gathered vector is the local one).
• Already correct: s_mpi_allreduce_min_vec, s_mpi_allreduce_vectors_sum, s_mpi_allreduce_integer_sum(_vec), s_mpi_reduce_int_sum, s_mpi_reduce_stability_criteria_extrema. s_mpi_reduce_min and s_mpi_reduce_maxloc are intent(inout), so the local value is already the result.

Affected callers on single-process builds: s_mpi_allreduce_min in m_boundary_common.fpp (global bounds), and s_mpi_allreduce_max/sum and s_mpi_gather_data in post_process m_data_output.fpp (probe/integral output). These now receive the local value instead of garbage.

Evidence

./mfc.sh test --no-mpi --no-gpu -j 24 --only IBM on Tuolumne (login node): 61 passed, 0 failed, 708 skipped. ./mfc.sh format and ./mfc.sh precheck pass.

Not verified: no dedicated test exercises these routines in a no-MPI build (the garbage read is not caught by the golden-file tests); the claim that callers now get the local value is from reading the code. MPI builds are unaffected (the change is inside #else), but I did not run MPI tests.

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Consolidated into draft PR #2024. This change is retained as a separate commit, and this original description is reproduced in full there. Original discussion remains available here.

[github-actions]

Lines of Code

File	Lines	Diff
src/common/m_mpi_common.fpp	1502	+8

Directory	Lines	Diff
common	10456	+8
total	47434	+8