bent-not-broken FTW (and BF FTW)! I loosened the stem and tightened it back down just snug this time, and all is right with the world. Front wheel flops freely when lifted. Walking the bike by the saddle exhibits no "carpet-squirreliness". A quick spin up and down the aisle of the parking lot yields perfectly upright hands-free riding (and a few seconds of getting used to a floppy front wheel again!!)
The reason it was so tight was that a friend of mine at work (who is a racer and mechanic from way back) noticed that my headset was loose. He said that happens after an install sometimes, riding awhile will cause the press-fit to set a little better and things get loose. So I torqued that mofo down HARD! I guess too hard.
Any suggestions on proper preload torque would be welcomed, but (a) I don't own a torque wrench and (b) "just snug" seems to be working great.
Thank you thank you everyone for all of your suggestions, even wheel-based ones which did not turn out to be the problem this time. The extra knowledge is added to my mental bag of tricks, and will surely come in handy some day!
One more edit: This is my ONE THOUSANDTH POST here at BF. I am so pleased that this momentous occasion marks all that I love about this community -- so many experts, so willing to share and help. Thank you all 1000 times!
Last edited by RubeRad; 06-12-13 at 12:05 PM.