Later, I tried to clean a small stain on it with cold water and mild hand soap. That stain was gone, but replaced by a much larger stain of the wetted area. Plain soap and water doesn't stain cotton, I thought, so I tried again with barely tepid water and a very little washing-up liquid. That has always worked well elsewhere, but here it just enlarged the stain to the new, bigger wetted area. It seems that the soap and water has reacted with the residue of the dry cleaning solvent.
Anyone know how to remove this stain short of sending it back to the dry cleaner?