Solution (source code)

= Solution
{c}

A <stationary distribution> $\pi=(\pi_1,\pi_2)$ satisfies $\pi P=\pi$ and $\pi_1+\pi_2=1$. The first equation reduces to the balance relation $a\pi_1=b\pi_2$. Since $a,b>0$, normalization gives the unique solution
$$
\boxed{\pi=\left(\frac b{a+b},\frac a{a+b}\right).}
$$