The craft-ecproj-format pin moves to storytold/craft-libs#409's new
head, where Head::read is the one reader and saved_by() is removed.
effectcraft-project's saved_by reads savedBy with Head::read, and from
ReadError::TooNew for a project with a newer schema.
saved_by now returns None for text that is not a JSON object with a u32
schema. EffectCraft calls it only on files Project::from_json opened,
so opening and importing projects are unchanged.