Add per-DoF mimic actuator mode

[jslee02]

• add per-DoF mimic actuator configuration and update mimic constraint handling
• guard coupler constraints for partial/mixed mimic joints to avoid null derefs; fall back to mimic motors otherwise
• surface gz-physics FAKE_INSTALL build logs when the nested build fails to ease CI debugging

Closes #1684.

API changes (with example)

Before (joint-wide mimic only):

joint->setActuatorType(dynamics::Joint::MIMIC);
joint->setMimicJoint(reference);
joint->setMimicMultiplier(2.0);
joint->setMimicOffset(0.1);

After (per-DoF mimic configuration):

joint->setActuatorType(1, dynamics::Joint::MIMIC);
dynamics::MimicDofProperties props;
props.mReferenceJoint = reference;
props.mReferenceDofIndex = 0;
props.mMultiplier = 2.0;
props.mOffset = 0.1;
joint->setMimicDofProperties({props});

Coupler constraints now activate only when every DoF is mimic with a valid reference; mixed joints fall back to mimic motors.

---

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 62.28070% with 86 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.10%. Comparing base (7160371) to head (b38a48b).
⚠️ Report is 266 commits behind head on main.

Files with missing lines	Patch %	Lines
dart/dynamics/Joint.cpp	44.76%	58 Missing ⚠️
dart/dynamics/detail/GenericJoint.hpp	70.68%	17 Missing ⚠️
dart/dynamics/Skeleton.cpp	53.84%	6 Missing ⚠️
dart/constraint/CouplerConstraint.cpp	80.00%	3 Missing ⚠️
dart/constraint/ConstraintSolver.cpp	91.30%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2222      +/-   ##
==========================================
+ Coverage   61.05%   61.10%   +0.04%     
==========================================
  Files         353      353              
  Lines       32144    32228      +84     
  Branches     4183     4225      +42     
==========================================
+ Hits        19627    19693      +66     
- Misses      12517    12535      +18     

Flag	Coverage Δ
unittests	61.10% <62.28%> (+0.04%)	⬆️

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

Files with missing lines	Coverage Δ
dart/constraint/JointConstraint.cpp	79.38% <100.00%> (+0.18%)	⬆️
dart/constraint/MimicMotorConstraint.cpp	81.69% <100.00%> (+4.45%)	⬆️
dart/dynamics/Joint.hpp	100.00% <ø> (ø)
dart/dynamics/detail/JointAspect.hpp	100.00% <ø> (ø)
dart/constraint/ConstraintSolver.cpp	79.92% <91.30%> (+0.45%)	⬆️
dart/constraint/CouplerConstraint.cpp	77.04% <80.00%> (-0.26%)	⬇️
dart/dynamics/Skeleton.cpp	76.48% <53.84%> (-0.06%)	⬇️
dart/dynamics/detail/GenericJoint.hpp	78.98% <70.68%> (+3.28%)	⬆️
dart/dynamics/Joint.cpp	66.95% <44.76%> (-9.64%)	⬇️

... and 2 files with indirect coverage changes

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[jslee02]

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Codex Review: Didn't find any major issues. Chef's kiss.

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