layoutParentDataProtocol

Creates a stable token for parent data a Layout policy reads through Measurable.parentData.

Pass the token to Layout as Layout.parentDataProtocol, and use that same instance from every org.jetbrains.compose.swing.layout.ParentDataModifier the policy understands. A token accepts only the policy layout that was built with that exact instance, so a declaration hoisted under another custom layout is refused before Swing attaches the child.