Remove the fluid center-of-mass output, which no case can ask for or turn off

[sbryngelson]

s_derive_center_of_mass computes each fluid's mass, mass-weighted position and total volume every timestep and writes them to fluid<i>_com.dat. There is no com_wrt parameter — the feature has no switch anywhere in toolchain/mfc/params/, so no case can request it and no case can turn it off. It is instead gated on probe_wrt:

if (proc_rank == 0 .and. probe_wrt) then
    call s_open_probe_files()
    call s_open_com_files()

So every probe-enabled case pays for a full-domain reduction per step — three to five GPU_ATOMIC accumulations per cell per fluid, then up to five MPI_ALLREDUCEs per fluid — and gets fluid<i>_com.dat files in the case directory that nobody asked for. Nothing in examples/, tests/, benchmarks/ or the docs reads them.

Removed: s_open_com_files, s_write_com_files, s_close_com_files, s_derive_center_of_mass, the c_mass array with its GPU_DECLARE/ALLOCATE/DEALLOCATE, and the four call sites. m_derived_variables no longer needs m_mpi_proxy — the allreduces were its only use of it.

Not touched: s_compute_centroid_offset in m_ibm.fpp, whose local center_of_mass is the immersed-body centroid. Different quantity, and it is used.

248 lines net removed. Goldens are unaffected — the CoM files were written to the case root, not D/.

Verified: ./mfc.sh precheck clean; CPU build; 25/25 on the probe, hypoelastic and IBM tests (the probe case 18DB27D5 is the only one that reached this code), and no *_com.dat is produced.

[Copilot]

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Pull request overview

Removes an always-on (when probe_wrt is enabled) center-of-mass (CoM) reduction + file output pipeline that had no user-facing parameter to enable/disable it, eliminating unnecessary per-timestep GPU atomics and MPI reductions and stopping unexpected fluid<i>_com.dat outputs.

Changes:

• Removed CoM file lifecycle routines (s_open_com_files, s_write_com_files, s_close_com_files) and their call sites.
• Removed the CoM reduction routine (s_derive_center_of_mass) and associated c_mass storage/allocation.
• Dropped m_mpi_proxy dependency from m_derived_variables since CoM allreduces were its only use there.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/simulation/m_derived_variables.fpp	Removes CoM derivation + I/O calls and simplifies finalization to only close probe files when enabled.
src/simulation/m_data_output.fpp	Removes CoM-related public APIs, c_mass storage, and CoM file open/write/close implementations.

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[github-actions]

Lines of Code

File	Lines	Diff
src/simulation/m_derived_variables.fpp	302	-139
src/simulation/m_data_output.fpp	1306	-61

Directory	Lines	Diff
simulation	27889	-200
total	46163	-200

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 61.35%. Comparing base (dc0aec1) to head (0ab85c0).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/simulation/m_derived_variables.fpp	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1873      +/-   ##
==========================================
+ Coverage   61.26%   61.35%   +0.08%     
==========================================
  Files          84       84              
  Lines       22330    22215     -115     
  Branches     3265     3254      -11     
==========================================
- Hits        13680    13629      -51     
+ Misses       6207     6151      -56     
+ Partials     2443     2435       -8     

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