Cap Mask contrast factor at maximum contrast

[jonoomph]

-Raise the Mask contrast denominator floor from 0.00001 to 0.5, capping the contrast factor at 40. This prevents contrast values near 20 from turning the 8-bit gray lookup into a one-value step function, where tiny mask image differences can create hard transparent edge artifacts during gradient wipes. -Add a regression test covering the maximum contrast transition band.

[jonoomph]

Ruler Time: Editor

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.76%. Comparing base (118fc2d) to head (ce560f1).
⚠️ Report is 5 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1067      +/-   ##
===========================================
+ Coverage    68.25%   68.76%   +0.51%     
===========================================
  Files          235      238       +3     
  Lines        28014    28583     +569     
===========================================
+ Hits         19121    19656     +535     
- Misses        8893     8927      +34     

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