Fix FreeJoint world Jacobian translation

[jslee02]

Fixes #1657.

This updates dynamics::FreeJoint Jacobian/integration logic to remain consistent when T_parentBodyToJoint and T_childBodyToJoint are non-identity.

• Fix FreeJoint::integratePositions() to integrate the relative transform (body twist) and solve back for Q, preserving correct motion with offset joint frames.
• Fix FreeJoint::getRelativeJacobianStatic(positions) to use the passed positions (instead of a cached Jacobian).
• Ensure setRelativeSpatialVelocity() uses the up-to-date pose-dependent Jacobian; fix updateRelativeJacobianTimeDeriv() for non-identity child joint frames.
• Add C++ integration coverage: FD world Jacobian + classic-deriv checks (including near-π), pose-change setter regression, and energy conservation checks.
• Add GUI example free_joint_cases (ImGui) to visualize linear-only/angular-only/both and edge cases, with built-in numeric checks.

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• [ ] Run pixi run test-all to lint, build, and test your changes (CI passed; locally ran pixi run lint, pixi run test, pixi run test-py)
• [x] Add unit tests for new functionality
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• [ ] Add Python bindings (dartpy) if applicable

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 62.93%. Comparing base (52f71d7) to head (59a9642). :warning: Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2298      +/-   ##
==========================================
+ Coverage   62.89%   62.93%   +0.03%     
==========================================
  Files         363      363              
  Lines       33023    33044      +21     
  Branches     4349     4347       -2     
==========================================
+ Hits        20769    20795      +26     
+ Misses      12254    12249       -5     

Flag	Coverage Δ
unittests	62.93% <100.00%> (+0.03%)	:arrow_up:

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Files with missing lines	Coverage Δ
dart/dynamics/FreeJoint.cpp	80.00% <100.00%> (+0.43%)	:arrow_up:

... and 3 files with indirect coverage changes

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[jslee02]

@codex review

[chatgpt-codex-connector[bot]]

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

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[jslee02]

@codex review

[jslee02]

@codex review

[jslee02]

@codex review

[chatgpt-codex-connector[bot]]

Codex Review: Didn't find any major issues. Can't wait for the next one!

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