Well, I got adventurous... I usually avoid the "more force is better" approach, but this time I decided to give it a shot. I got a little plastic sleeve and put it over the shift cable where it first meets the top tube of the frame, close-ish to the shifter. Then I clamped down with some pliers and gave it a decent tug. Nothing happened for a few attempts, but finally there was some give, the release lever released, and the takeup lever started working again. I went through several cycles of this and finally got all the expected clicks in both directions. I did that a few more times and then reassembled and tuned... now it's working again. I assume that either the cable was somehow catching or grimed up (which seems unlikely on a clean 3-month old bike) or it's haunted. Calling Dr. Venkman tomorrow. Thanks for the replies... they definitely sparked the idea.