Got it out!
Here's what worked and what didn't work:
Worked: Grabbed the top tube in the workstand clamp close to the head tube. Loosened stem bolt and whacked on it for all I was worth with a heavy hammer. After a few minutes it started to move. Used a 3/8" bronze rod for a punch so I didn't bung up the bolt head. Backed the bolt out a bit and repeated until I was down to a few threads.
Didn't work: removed the stem bolt, turned the bike over and pounded on the bottom of the stem close to the headset, using a long 1/2" bronze rod as a punch. No movement. Pounded on the cylinder part of the wedge with a 5/16" rod and heavy hammer. No movement.
Worked: Demounted bars, and heated stem close to headset with a propane torch, wide flame, for quite a while, until the metal near the carbon fork crown started to get warm. Heard a few cracking noises while doing this, good. Of course this ruined the headset, but I had no problem with that. Then poured cold water down the inside of the stem to cool it. Lots more crackling. Pounded on the wedge again from the bottom, with the 5/16" rod and heavy hammer. This time it moved about 1/16" after several minutes of pounding. After it moved, I put the bars back on the stem and remounted the wheel and torqued on the bars. The stem moved! Then there was nothing to it - came right out.
The corrosion was just at the top of the stem and the wedge. Most of the length of the stem never got Kroil on it, and still had some anti-seize on it, very dry and dirty. I think the main effect of the heating was to make the stem longer, and longer faster than the steer tube, partly because aluminum conducts heat so well and was not well bonded to the steer tube. That broke it loose so that pounding could move it. Once it moved, it was easy.