DependencyChange

Package name exactly as uv reported it in the diff line.

Version before the change; None when the package was newly added.

Version after the change; None when the package was removed.

Classify the change from which version sides are present.

Reading the case from here keeps consumers (e.g. format_dependency_changes) from re-deriving it via field truthiness, and turns the meaningless (None, None) shape into a hard error instead of silently rendering as a removal.