And it pegged out the bench supply. Why didn't anyone tell me the Vcc and GND lines were switched on ALL THREE 744066 chips? I was seeing a diode drop of 0.6V between 5V and GND so I went in a loop around the whole board cutting power traces until it went away; it wasn't until I was about to cut on the third 4066 that I realized it didn't make a lot of sense for the Vcc to come in pin 7 on a 14-pin logic.
Somehow in the schematic layout the symbols got flipped for those switches, where the +V was on bottom and -V was on top, and I didn't catch it when connecting the Vcc and GND to everything in schematic layout. Oh well. At least that's only needed for the residual calculation, which I technically don't need until I have a second board - but which needs to be successfully tested. I'll reconnect those pins to the proper rails manually after successfully testing the divider ladder and logic output. More news soon.