Handle unset VisualAspect color separately

[jslee02]

• Track whether a VisualAspect color is unset vs explicit (usesDefaultColor/resetColor/getDefaultRGBA)
• Treat textured meshes as neutral when the color is unset while still allowing explicit tinting and alpha control
• Add unit coverage and register the new VisualAspect test

Fixes #739

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• Add unit tests for new functionality
• Document new methods and classes
• Add Python bindings (dartpy) if applicable

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.82%. Comparing base (a3390cd) to head (88d33be).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2230      +/-   ##
==========================================
+ Coverage   60.77%   60.82%   +0.04%     
==========================================
  Files         353      353              
  Lines       31866    31880      +14     
  Branches     4128     4129       +1     
==========================================
+ Hits        19368    19392      +24     
+ Misses      12498    12488      -10     

Flag	Coverage Δ
unittests	60.82% <100.00%> (+0.04%)	⬆️

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Files with missing lines	Coverage Δ
dart/dynamics/ShapeFrame.cpp	78.47% <100.00%> (+7.15%)	⬆️
dart/dynamics/ShapeFrame.hpp	75.00% <ø> (ø)
dart/dynamics/detail/ShapeFrameAspect.hpp	100.00% <ø> (ø)

... and 2 files with indirect coverage changes

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