Mostly because I wanted to verify that it really worked:
The Arduino Nano runs the default Blink program that all the knockoff manufacturers use as their final QC test.
The MP1584 specs say the Enable input can accept a logic signal up to 6 V, the Nano runs at 5 V regulated down from the 6.3 V from the bench supply, and the 1 W red LED now flashes 1 s ON / 1 s OFF.
The current feedback works as it did before, too, which is comforting.
The Nano adds 20 mA to the bench supply, so the whole affair runs at 220 mA = 1.4 W. Of course, it’s now at a 50% duty cycle, so that helps.