Solution (source code)

= Solution

At the crossing the angular frequency is $\omega_c=1$, so the limiting small-amplitude period is
$$
\boxed{T_c=\frac{2\pi}{\omega_c}=2\pi=4\tau_c.}
$$
This is the period of the bifurcating infinitesimal oscillation. Finite-amplitude oscillations away from the threshold need not have period exactly $4\tau$.