Implement WeldJoint merge

[jslee02]

• add WeldJoint::merge() to flatten a welded child into its parent while combining inertia, preserving external forces, cloning attached nodes, and reparenting descendants without disturbing world transforms
• expose BodyNode::getNodes()/const variant so WeldJoint can clone attached nodes, and allow WeldJoint to clone nodes via friendship

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Before creating a pull request

• Run pixi run test-all to lint, build, and test your changes
• Add unit tests for new functionality
• Document new methods and classes
• Add Python bindings (dartpy) if applicable

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.87%. Comparing base (5237903) to head (04e5eb7).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
dart/dynamics/WeldJoint.cpp	86.66%	6 Missing ⚠️
dart/dynamics/BodyNode.cpp	50.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2242      +/-   ##
==========================================
+ Coverage   60.82%   60.87%   +0.04%     
==========================================
  Files         353      353              
  Lines       31880    31935      +55     
  Branches     4128     4137       +9     
==========================================
+ Hits        19392    19440      +48     
- Misses      12488    12495       +7     

Flag	Coverage Δ
unittests	60.87% <80.00%> (+0.04%)	⬆️

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

Files with missing lines	Coverage Δ
dart/dynamics/BodyNode.hpp	0.00% <ø> (ø)
dart/dynamics/Node.hpp	100.00% <ø> (ø)
dart/dynamics/WeldJoint.hpp	100.00% <ø> (ø)
dart/dynamics/BodyNode.cpp	84.33% <50.00%> (+0.05%)	⬆️
dart/dynamics/WeldJoint.cpp	86.30% <86.66%> (+0.58%)	⬆️

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