GitHub social preview is broken

Earlier today, now that Rogallo has matured a fair bit, I thought I'd update the banner image and, in doing so, refresh the "social preview" for the repository . This is easy enough, and something I've done plenty of times with plenty of projects. You just go into the general settings and upload a new image. So, I faffed in Pixelmator Pro for a short while, got the new banner going, and then…