Refactor: Move rounding to promotion calculator module

[mamhoff]

Summary

All promotion calculators need to be able to round correctly, and we can centralize that logic.

Extracted from #6287

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.35%. Comparing base (d070f94) to head (02d04da).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6332   +/-   ##
=======================================
  Coverage   89.35%   89.35%           
=======================================
  Files         961      961           
  Lines       20195    20195           
=======================================
  Hits        18046    18046           
  Misses       2149     2149           

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