At last, I’m getting valid keyboard input.
What was wrong? It’s so embarrassing. I had a 21.5KΩ 1% resistor out of place. It was supposed to be part of a voltage divider on one of the input pins of the LM311 comparator. Instead, it was just hanging out doing nothing. So, why was I getting ANY input at all? I’m sure that if I dug in I could analyze the circuit and figure out exactly why it was almost-but-not-quite-working without that resistor, but frankly I’m just glad to have sorted it out! I’ll leave it as an exercise to the reader.
So, with that taken care of, I can get back to writing the firmware. Again, DEC actually has a pretty good explanation of how their firmware works, so I’m going to try to get mine to behave similarly.