Fix joint impulse statefulness

[jslee02]

Fixes part of #1089 by moving GenericJoint impulse-related members into the joint Aspect State so they are captured/restored by dart::dynamics::Skeleton::getState() / setState().

• Adds mVelocityChanges, mImpulses, and mConstraintImpulses to dart::dynamics::detail::GenericJointState and updates GenericJoint/FreeJoint to use them.
• Makes dart::simulation::World::reset() clear constraint impulses so stale constraint forces don't leak across independent runs.
• Adds regression tests in tests/integration/dynamics/test_SkeletonState.cpp.
• Updates GitHub CI workflows to pin the pixi install location (avoids intermittent pixi: command not found / exit 127 on some runners).

Related: #1086.