Improve ShapeNode inertia coverage

[jslee02]

Fixes #348.

Add regression coverage for the new ShapeNode inertia helpers so BodyNode aggregation and parser importers are tested end-to-end, plus document the brace style that protects these paths. Includes two fixtures to exercise both .skel and .vsk importers.

• add unit tests for ShapeNode density conversion and invalid mass providers
• add Skel/Vsk integration tests driven by new fixtures
• update onboarding docs with the "always use braces" reminder

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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

[jslee02]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Delightful!

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[jslee02]

@codex review

[chatgpt-codex-connector]

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

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.69%. Comparing base (038cc73) to head (4630b41).
⚠️ Report is 332 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2187      +/-   ##
==========================================
+ Coverage   63.65%   63.69%   +0.04%     
==========================================
  Files         394      394              
  Lines       33362    33362              
  Branches     4162     4163       +1     
==========================================
+ Hits        21235    21249      +14     
+ Misses      12127    12113      -14     

Flag	Coverage Δ
unittests	63.69% <ø> (+0.04%)	⬆️

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see 4 files with indirect coverage changes

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