This panel belongs to the PLC. Everything on it acts on the device
directly. Nothing here talks to the middleware.
Light barriers
A light barrier is a sensor. The middleware reads it and cannot move it,
so these two buttons stand in for a workpiece. They call
transfer_unit.py → set_occupied().
The throughput button starts a loop that cycles the barriers automatically
while a belt runs. It calls transfer_unit.py → set_throughput_simulation().