Numerical Methods: Euler to Runge–Kutta · Challenge problem

Heun's method applied to the test equation $y' = \lambda y$ gives $y_{n+1} = \left(1 + z + \dfrac{z^2}{2}\right) y_n$ with $z = \lambda h$. For $\lambda = -10$, what is the largest step size $h$ for which the iterates do not grow, that is, $\left|1 + z + \dfrac{z^2}{2}\right| \le 1$? Express your answer as a common fraction.