Handle aiScene ownership with shared_ptr custom deleters

[jslee02]

Summary

• manage aiScene meshes in MeshShape via shared_ptr with explicit ownership and custom deleters
• update loaders, collision pipelines, GUI, and examples to propagate shared meshes without double frees
• add regression coverage for custom deleters and patch downstream bindings/tools for the new semantics

Fixes #452.

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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 (n/a)
• Add Python bindings (dartpy) if applicable

[codecov]

Codecov Report

❌ Patch coverage is 66.15385% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.68%. Comparing base (f891d7e) to head (bcd1b68).
⚠️ Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
dart/dynamics/MeshShape.cpp	71.66%	17 Missing ⚠️
dart/dynamics/ArrowShape.cpp	0.00%	3 Missing ⚠️
dart/dynamics/ReferentialSkeleton.cpp	0.00%	1 Missing ⚠️
dart/dynamics/Skeleton.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2285      +/-   ##
==========================================
- Coverage   61.70%   61.68%   -0.03%     
==========================================
  Files         360      360              
  Lines       32520    32618      +98     
  Branches     4271     4281      +10     
==========================================
+ Hits        20067    20119      +52     
- Misses      12453    12499      +46     

Flag	Coverage Δ
unittests	61.68% <66.15%> (-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/MetaSkeleton.hpp	100.00% <ø> (ø)
dart/dynamics/ReferentialSkeleton.hpp	100.00% <ø> (ø)
dart/dynamics/Skeleton.hpp	100.00% <ø> (ø)
dart/dynamics/ReferentialSkeleton.cpp	44.54% <0.00%> (ø)
dart/dynamics/Skeleton.cpp	77.48% <0.00%> (-0.18%)	⬇️
dart/dynamics/ArrowShape.cpp	0.00% <0.00%> (ø)
dart/dynamics/MeshShape.cpp	73.85% <71.66%> (-3.81%)	⬇️

... and 1 file with indirect coverage changes

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