New master came today. Cardone 11-2585, which the Rockauto catalog says is correct for four-wheel disk. The picture on Amazon showed a reworked factory casting, but of course that's not what I got. The new one came in a Cardone box with the right PN on it, but is clearly a new aftermarket part. It looks almost exactly like the last one, right down to having "AB-36" cast into the side (though it is in a different place).
I bled it (the hoses they include for this are useless, so I went back to plugs), installed it, did some bleeding, and the pedal feels exactly the same as before.
Either I've bought another defective or incorrect master cylinder, or the master cylinder was never the problem.
I stumbled on this a while ago and thought, nah, can't be that. I'm running out of ideas and I'm sick and tired of brake fluid, so tomorrow, the booster's coming off, and I'm going to shake it and see if pieces fall out.