This type of game is up my alley, so I played it to the end. It was good fun, but I do have some critique.
I wish I could see the $SENSOR the way I see other registers. I basically didn't need to use the registers: the reason to use them is loops, but they are more effort to write in assembly than writing move the amount of times you need, and since the robot waits on a non-move command it's faster to do it that way too. If anything, I wish I could just write "move 5" and be done with it. The levels did not actually require behaviour that was that complex.
Anyway, overall you made a great programming game. Reminds me of messing around with turtles in modded Minecraft :)