Document FreeJoint velocity actuator workflow

[jslee02]

Summary

• synchronize FreeJoint spatial velocity/acceleration setters with the generic joint state so VELOCITY/ACCELERATION actuators stay aligned (closes Add Kinematic Joint #1953)
• document the behavior in FreeJoint.hpp and add a regression test that drives a VELOCITY-actuated FreeJoint purely via setSpatialVelocity
• show that the existing FreeJoint covers the Gazebo workflow the original PR proposed without introducing a new joint type

Testing

• pixi run lint
• pixi run test -- tests/integration/dynamics/test_Joints.cpp
• pixi run -e gazebo test-gz

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.48%. Comparing base (dd6f423) to head (d4c14a3).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2112   +/-   ##
=======================================
  Coverage   61.48%   61.48%           
=======================================
  Files         393      393           
  Lines       33155    33157    +2     
  Branches     4084     4085    +1     
=======================================
+ Hits        20384    20386    +2     
+ Misses      12771    12770    -1     
- Partials        0        1    +1     

Flag	Coverage Δ
unittests	61.48% <100.00%> (+<0.01%)	⬆️

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

Files with missing lines	Coverage Δ
dart/dynamics/FreeJoint.cpp	75.91% <100.00%> (+0.17%)	⬆️
dart/dynamics/FreeJoint.hpp	100.00% <ø> (ø)

... and 1 file with indirect coverage changes

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