Add state-independent Joint::integratePositions

[jslee02]

Fixes #699.

• Add state-independent Joint position integration overloads.
• Implement for GenericJoint, BallJoint, FreeJoint, and ZeroDofJoint.
• Refactor IK Jacobian gradient conversion to avoid mutating Skeleton velocities/positions.
• Add regression tests for stateless integration + IK gradient conversion.

[codecov]

Codecov Report

❌ Patch coverage is 86.88525% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.72%. Comparing base (0c187b1) to head (1082518).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
dart/dynamics/BallJoint.cpp	80.00%	2 Missing ⚠️
dart/dynamics/FreeJoint.cpp	81.81%	2 Missing ⚠️
dart/dynamics/ZeroDofJoint.cpp	60.00%	2 Missing ⚠️
dart/dynamics/detail/GenericJoint.hpp	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2309      +/-   ##
==========================================
+ Coverage   62.68%   62.72%   +0.03%     
==========================================
  Files         361      361              
  Lines       32820    32866      +46     
  Branches     4317     4322       +5     
==========================================
+ Hits        20574    20615      +41     
- Misses      12246    12251       +5     

Flag	Coverage Δ
unittests	62.72% <86.88%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
dart/dynamics/BallJoint.hpp	100.00% <ø> (ø)
dart/dynamics/FreeJoint.hpp	100.00% <ø> (ø)
dart/dynamics/GenericJoint.hpp	100.00% <ø> (ø)
dart/dynamics/HierarchicalIK.cpp	54.54% <ø> (-0.40%)	⬇️
dart/dynamics/InverseKinematics.cpp	54.62% <100.00%> (+0.42%)	⬆️
dart/dynamics/Joint.cpp	67.32% <100.00%> (+0.37%)	⬆️
dart/dynamics/Joint.hpp	100.00% <ø> (ø)
dart/dynamics/ZeroDofJoint.hpp	100.00% <ø> (ø)
dart/dynamics/BallJoint.cpp	81.81% <80.00%> (-0.33%)	⬇️
dart/dynamics/FreeJoint.cpp	79.56% <81.81%> (+0.09%)	⬆️
... and 2 more

... and 1 file with indirect coverage changes

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[jslee02]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🎉

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