Fix joint impulse statefulness

[jslee02]

Fixes part of #1089 by moving GenericJoint impulse-related members into the joint Aspect State so they are captured/restored by dart::dynamics::Skeleton::getState() / setState().

• Adds mVelocityChanges, mImpulses, and mConstraintImpulses to dart::dynamics::detail::GenericJointState and updates GenericJoint/FreeJoint to use them.
• Makes dart::simulation::World::reset() clear constraint impulses so stale constraint forces don't leak across independent runs.
• Adds regression tests in tests/integration/dynamics/test_SkeletonState.cpp.
• Updates GitHub CI workflows to pin the pixi install location (avoids intermittent pixi: command not found / exit 127 on some runners).

Closes: #1089.

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.63%. Comparing base (a6b93bf) to head (a598e07).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
dart/dynamics/detail/GenericJoint.hpp	86.95%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2308      +/-   ##
==========================================
+ Coverage   64.58%   64.63%   +0.05%     
==========================================
  Files         367      367              
  Lines       33826    33837      +11     
  Branches     4472     4474       +2     
==========================================
+ Hits        21845    21872      +27     
+ Misses      11981    11965      -16     

Flag	Coverage Δ
unittests	64.63% <90.00%> (+0.05%)	⬆️

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

Files with missing lines	Coverage Δ
dart/dynamics/GenericJoint.hpp	100.00% <ø> (ø)
dart/dynamics/detail/GenericJointAspect.hpp	71.42% <100.00%> (+1.27%)	⬆️
dart/simulation/World.cpp	79.27% <100.00%> (+1.58%)	⬆️
dart/simulation/World.hpp	100.00% <ø> (ø)
dart/dynamics/detail/GenericJoint.hpp	79.56% <86.95%> (+0.36%)	⬆️

... and 4 files with indirect coverage changes

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