b (source code)
= b
{scope}
Let $S'$ be the runner's <inertial frame>, with origin at the entry event. The <Lorentz transformation> is
$$
x'=\gamma(x-vt),\qquad t'=\gamma\left(t-\frac{vx}{c^2}\right),\qquad\gamma=(1-\beta^2)^{-1/2}.
$$
The emission event $E=(L,0)$ has coordinates $x'_E=\gamma L$, $t'_E=-\gamma vL/c^2$. It is earlier than entry in $S'$: entry and far-end emission are simultaneous only in the tunnel frame. This is <relativity of simultaneity>, not an extra delay of the light source.
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