Function analytical_step_jacobians 

pub fn analytical_step_jacobians(model: &Model, state: &State) -> StepJacobians

Compute analytical step Jacobians for semi-implicit Euler.

Semi-implicit Euler: qdd = ABA(q, v, ctrl) v’ = v + dt * qdd q’ = q + dt * v’

Derivatives via chain rule through ABA. Uses finite differences on ABA itself for now (analytical ABA derivatives in future PR).