Fix loc_violations used uninitialized in MPI_Allreduce

[sbryngelson]

Summary

Severity: HIGH — non-violating MPI ranks sum garbage in the reduction.

File: src/pre_process/m_data_output.fpp, line 479

loc_violations is declared but never initialized to 0. It is only assigned inside a conditional (if (mod(...) > 0)), so ranks that don't enter the conditional contribute uninitialized values to the subsequent MPI_Allreduce sum.

Before

real(wp) :: loc_violations, glb_violations
...
if (down_sample) then
    if ((mod(m + 1, 3) > 0) .or. ...) then
        loc_violations = 1._wp     ! only set inside this branch
    end if
    ! MPI_Allreduce sums loc_violations across ranks — garbage if branch not taken

After

real(wp) :: loc_violations = 0._wp, glb_violations

Why this went undetected

Ranks that happen to have zero-initialized stack memory would produce correct results. The bug is non-deterministic and depends on memory state.

Test plan

• Run multi-rank pre-process with downsampling enabled
• Verify violation count is deterministically correct

🤖 Generated with Claude Code

Fixes #1206

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Pull request overview

This PR fixes a bug where the loc_violations variable is used in an MPI_Allreduce operation without being initialized, causing non-violating ranks to contribute undefined values to the global sum.

Changes:

• Initialize loc_violations to 0 at declaration to prevent uninitialized memory from being used in MPI reduction operations

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 44.05%. Comparing base (84c46e0) to head (1f03190).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1186   +/-   ##
=======================================
  Coverage   44.05%   44.05%           
=======================================
  Files          70       70           
  Lines       20498    20499    +1     
  Branches     1990     1990           
=======================================
+ Hits         9030     9031    +1     
  Misses      10329    10329           
  Partials     1139     1139           

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