Well guys, I'm losing my effing mind.

Removed the (new, recently-replaced) master cylinder. The front circuit seal had indeed failed completely - it would not build any pressure on the bench. Sent it back for a refund.

Got a replacement master cylinder. It built pressure on both circuits on the bench while bench bleeding, and I was very careful to not over depress the piston in case this could damage a seal. OK, so far so good. Installed it, thoroughly bled the system (pressure, DSC, and manual). Pedal feels firm and holds when vehicle is not running. When the vehicle is running (brake booster on) with firm pedal pressure the pedal will slowly sink 80% to the floor which is the same symptom I had previously where the front circuit seal had failed. Pumping the brakes firms things up, but the pedal will sink to 80% if you continue to press hard. I suspect that this 2nd new master cylinder has a leaking but not outright failed front seal.

Again, no bubbles, no leaks.

Unless someone has any insight, I'm going to order an OE MC from FCP Euro. If that works then I'm going to disassemble the Beck/Arnley MC from RockAuto to see what is wrong with the seal.