Class hierarchy of Transition vs Item vs Composable not quite right.

There are a few methods, like `range_in_parent` that are implemented on Transition, but really belong to Item. Should Transition be an Item or should those methods move to Composable?

Review the Transition, Item, and Composable class definitions and the range_in_parent implementations mentioned in the issue. Determine the intended ownership and inheritance relationship, then confirm the agreed design with maintainers before changing it; done means the hierarchy and method placement are consistent without changing existing behavior.

Written by the indexing model from the issue text.