Based on the pain today, I think I've strained the upper part of the sartorius muscle including its attachment point to the pelvis. The pain is front upper thigh, hip crease and the attachment point on the outside of the pelvis, and I don't notice it unless I start into a reverse lunge (which I obviously won't finish!), take a long stride when walking or sit cross-legged ("tailor's position", the origin of the muscle's name).
As sports injuries go, it's not particularly sore - no swelling or bruising visible. I seem to be able to move all my body parts in all the usual directions. Short indoor jogs seem fine; I'm going to try snowshoeing before I run. I think running downhill would trigger the pain.
All of this seems encouraging except that it doesn't explain WTF happened yesterday. FB's idea is not as far-fetched as I'd like, given what I felt. My right hip has always been less able than my left hip to rotate to the outside. After years of yoga, chiro and stretches, I've just chalked it up to asymmetric anatomy. That hasn't changed since yesterday; I still have trouble rotating my right hip all the way to the side in yoga tree pose but my left hip can do it.
I'm hoping that something has come "unstuck" in a good way or that some piece of soft tissue snapped across some bone harmlessly. I'll aim to ask Dr. Thumbs-of-Death soon. It's always best to see him after the inflammation goes down.