What to build

Integrate a crate like num-dual or autodiff to provide exact Jacobians automatically in ODE::jacobian, replacing the default finite-difference approximation for better performance in stiff solvers.

Acceptance criteria

• ODE trait provides an optional AD-based Jacobian implementation.
• BDF and ImplicitRungeKutta solvers benefit from exact Jacobians.
• Comparison tests verifying AD accuracy against finite differences.

Blocked by

None - can start immediately