The corollary of ‘measure twice, cut once’ for computer use is something like, ‘README twice, click ‘upgrade automatically?” once.
I knew — somewhere, just not front and center — that the auto-upgrade feature of WordPress would overwrite any code you may have inserted into the ‘classic’ or ‘default’ themes. So, the sidebar and footer are ‘default’ now.
I knew that.
I think I have backups around here, somewhere… (/rolls eyes)