= Solution
<Interference> <CP violation> concerns the relative <phase> between direct decay and decay after mixing. For a common <CP eigenstate> final state, it can occur with both $|q/p|=1$ and $|\bar A_f/A_f|=1$, so neither of the first two mechanisms is needed. In that limit $|\lambda|=1$, but
$$
\boxed{\operatorname{Im}\lambda\ne0}
$$
produces different <interference> terms for $\lambda$ and $\lambda^{-1}=\lambda^*$. With $\Delta m=m_h-m_\ell$,
$$
g_-g_+^*=\frac14\left[e^{-\Gamma_\ell t}-e^{-\Gamma_h t}+2i e^{-(\Gamma_\ell+\Gamma_h)t/2}\sin(\Delta m t)\right].
$$
The term proportional to $\operatorname{Im}\lambda\sin(\Delta m t)$ consequently reverses sign. In the CP-conserving limit, $\lambda$ is the real <CP> <parity> of the final state in a consistent <phase> convention. For non-CP-eigenstate final states, compare the full coefficients in CP-conjugate rates rather than imposing this simplified criterion.
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