Function unit_joint_friction_constraint 

pub fn unit_joint_friction_constraint(
    params: &IntegrationParameters,
    multibody: &Multibody,
    link: &MultibodyLink,
    friction: f32,
    dof_id: usize,
    j_id: &mut usize,
    jacobians: &mut DVector,
    constraints: &mut [GenericJointConstraint],
    insert_at: &mut usize,
    softness: SpringCoefficients<f32>,
)

Generates the dry-friction (MuJoCo frictionloss) velocity constraint for one generalized DoF.

softness supplies the CFM compliance (MuJoCo’s solreffriction); only CFM applies, never ERP, since there is no position error for a bias to chase.