Add non-autonomous stiff3 overload with dF/dx callback and regression test

[Copilot]

stiff3 only exposed an autonomous API, while the underlying ROS4 approach supports non-autonomous systems. This change adds a second stiff3 interface variant for F(x,y) that also accepts ∂F/∂x, without disrupting existing autonomous call sites.

• API extension (non-autonomous support)

  • Added an overloaded stiff3 procedure that accepts:
    • RHS: fun(n, x, y, f)
    • Jacobian wrt state: dfun(n, x, y, df)
    • Derivative wrt independent variable: dfdx(n, x, y, fx)
  • Preserved the current autonomous signature as-is.

• Implementation approach

  • Added a non-autonomous wrapper in stiff3_solver that lifts the system to an augmented autonomous form and reuses the existing integrator path.
  • Added dedicated abstract callback interfaces for non-autonomous RHS/Jacobian and dF/dx.

• Docs alignment

  • Updated README wording to reflect support for both autonomous and non-autonomous systems.

• Regression coverage

  • Added test/nonautonomous.f90 using a scalar non-autonomous ODE with closed-form solution to exercise the new API shape and verify endpoint accuracy.

call stiff3(n, fun, jac, dfdx, out, nprint, x0, x1, h0, eps, w, y)

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