perf: tighten growth factor threshold and improve documentation

[steven-murray]

• Introduced a low radiation threshold for growth factor calculations.
• Updated growth factor logic to use the new threshold.
• Enhanced tests to validate growth factor behavior at the new threshold.
• Added a utility script to determine safe radiation thresholds.
• Created documentation for the comparison between hmf and Colossus.

Fixes #294

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.83%. Comparing base (8ba41d1) to head (4d1c500).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #298      +/-   ##
==========================================
+ Coverage   97.79%   97.83%   +0.03%     
==========================================
  Files          26       26              
  Lines        2633     2634       +1     
==========================================
+ Hits         2575     2577       +2     
+ Misses         58       57       -1     

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