Fix NaN check reading wrong index in MPI unpack

[sbryngelson]

User description

Summary

• The Intel-only NaN diagnostic check after MPI buffer unpacking used q_comm(i)%sf(j, k, l) instead of q_comm(i)%sf(j + unpack_offset, k, l)
• The data is written to the offset index but the NaN check read from the non-offset index, checking the wrong cell

Test plan

• Intel-compiler-only diagnostic code path; no functional change on other compilers
• No golden file changes expected

🤖 Generated with Claude Code

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CodeAnt-AI Description

Fix NaN diagnostic to check unpacked MPI buffer locations

What Changed

• The Intel-only NaN check now inspects the exact array cell that was just written by MPI unpack (uses the unpack offset) instead of an unrelated cell
• Eliminates checks against stale or wrong elements after receiving MPI buffer data

Impact

✅ Fewer spurious NaN error stops during MPI unpack
✅ Clearer diagnostics that point to the just-received value
✅ More reliable MPI data validation on Intel compilers

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[cubic-dev-ai]

No issues found across 1 file

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

[Copilot]

Pull request overview

This PR fixes a bug in Intel compiler-specific diagnostic code where NaN checks after MPI buffer unpacking were reading from incorrect array indices. The data was being written to offset indices (j + unpack_offset, k + unpack_offset, or l + unpack_offset depending on the spatial direction), but the NaN checks were reading from non-offset indices (j, k, or l), effectively checking the wrong memory cells.

Changes:

• Fixed NaN check index in j-direction MPI buffer unpacking to use j + unpack_offset instead of j
• Fixed NaN check index in k-direction MPI buffer unpacking to use k + unpack_offset instead of k
• Fixed NaN check index in l-direction MPI buffer unpacking to use l + unpack_offset instead of l

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 44.05%. Comparing base (d048c4b) to head (1819d87).
⚠️ Report is 19 commits behind head on master.

Files with missing lines	Patch %	Lines
src/common/m_mpi_common.fpp	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1231      +/-   ##
==========================================
- Coverage   44.07%   44.05%   -0.02%     
==========================================
  Files          70       70              
  Lines       20431    20498      +67     
  Branches     1974     1990      +16     
==========================================
+ Hits         9004     9030      +26     
- Misses      10291    10329      +38     
- Partials     1136     1139       +3     

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[sbryngelson]

Superseded by #1242 (batched HPC-sensitive fixes)