At the speed-limit boundary, the toy diagram gives the moving leg the whole 5 m/s limit and leaves a quiet leg of 0. That zero leg is a model boundary check: it explains why no finite gamma is assigned there, while real particles are compared to measured speeds below the limit.
The v equals c boundary is represented by a zero quiet leg. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
At the limit the quiet leg vanishes
At v equals c, both are 5 m/s and the quiet leg is 0 m/s.
v=c=5q=0
No finite gamma remains
With quiet leg 0, the finite-gamma flag is rejected.