♻️ Refactor Bruner div heat split

[chris-ashe]

Description

Checklist

I confirm that I have completed the following checks:

• My changes follow the PROCESS style guide
• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 33.69565% with 122 lines in your changes missing coverage. Please review.
✅ Project coverage is 49.26%. Comparing base (feb7ef3) to head (95eb74f).

Files with missing lines	Patch %	Lines
process/core/io/plot/summary.py	4.81%	79 Missing ⚠️
process/models/physics/exhaust.py	55.17%	26 Missing ⚠️
process/models/physics/physics.py	0.00%	16 Missing ⚠️
process/models/build.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4511      +/-   ##
==========================================
- Coverage   49.32%   49.26%   -0.06%     
==========================================
  Files         150      150              
  Lines       29802    29931     +129     
==========================================
+ Hits        14700    14746      +46     
- Misses      15102    15185      +83     

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