Expose ndrop mixing substep count

[susburrows]

Expose the exact ndrop explicit mixing substep count without changing the physics.

Summary

• Add an int& nsubmix output to update_from_explmix and dropmixnuc.
• Return the same integer that directly controls the existing explicit mixing loop.
• Update all in-tree validation call sites with local sink variables, leaving their external schemas unchanged.
• Add a deterministic Kokkos device test for the returned count.

Validation

• ctest -R '^mam4_ndrop_unit_tests$' --output-on-failure: 1/1 passed.
• Complete ndrop-label suite: 78 passed and 2 existing max-saturation comparisons were skipped by their configured tolerance rule; no numerical failures.
• Independent review: PASS for draft-PR readiness.

Scope

This change does not alter the computed substep count, loop bounds, equations, limiters, state updates, or production physics.

[susburrows]

The purpose of this PR is to expose the number of ndrop explicit substeps for diagnostic output, to assist with understanding the performance impact of the explicit solver.

Developed with codex. FYI @odiazib @jeff-cohere @overfelt

[susburrows]

Used in E3SM-Project/E3SM#8645

[odiazib]

I don’t think we need a unit test for these diagnostic variables. While agentic coding makes it easy to add these tests, we also need to consider the long-term maintenance effort.

[susburrows]

That makes sense. I can remove it and push a revised version.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.03%. Comparing base (8fbf775) to head (0d076d8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #577      +/-   ##
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+ Coverage   93.01%   93.03%   +0.01%     
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  Files         311      311              
  Lines       24581    24631      +50     
  Branches     2828     2828              
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+ Hits        22865    22915      +50     
  Misses       1716     1716              

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[jeff-cohere]

@susburrows , here are a few simple "on/off" compile-time options we make available for mam4xx. You could add another one for a diagnostics-heavy eval-facing configuration. Basically:

1. Add another option alongside the existing ones (e.g. MAM4XX_EVAL_DIAGNOSTICS)
2. Edit mam4_config.hpp.in and add a line to pass the CMake option to the preprocessor (e.g. #cmakedefine MAM4XX_EVAL_DIAGNOSTICS)
3. Use the macro (MAM4XX_EVAL_DIAGNOSTICS) to enable/disable code as needed.