It helped that the urgent care wasn't especially crowded.
They gave me an EKG (and chastised me for not coming in yesterday): it turned out normal.
They didn't do a stress EKG, just resting. Instead, they did a blood test to look for the specific proteins that heart muscle releases when it's damaged. That, too, turned out normal. Whatever it is, it's not heart -- that "bruised esophagus" hypothesis is as likely as anything.
So! Nothing to worry about, nothing to even slow me down. Even Saturday's chest flutter wasn't a matter of concern -- just my body's way of saying "either slow the bleep down, or WALK MORE OFTEN."
This is really just some mild discomfort. It seems positional -- if I lean back, it's less of an issue.
Thanks for the concern and kind words in the last post, everyone.