It came to me just after I turned the light out last night: I think it's a Kerria japonica 'Pleniflora', so it is in the rose family after all. I have spotted one mention of a fungus from the Morton Arboretum in IL: A Septoria-like fungus...pretty darn destructive and fast-acting. Initial symptoms are brown leaf spots that spread quickly throughout the entire plant. Shortly after these symptoms appear, branch dieback begins.

Guess it doesn't matter much--I've cleaned out the dead stems so there's much better air circulation in there, so I'm just going to hope that the plant will come back. It's probably much too late to try applying an anti-fungal treatment.