Validate a bounded UI snapshot

Validate a complete snapshot for an intentionally tiny **local catalog**, not the entire A2UI standard catalog. Allowed exact properties are Text {id,component,text}, Column {id,component,children}, and Button {id,component,child,action} with action approve or reject. Require unique nonempty string IDs, root, valid property types, all references present, no cycles, and all declarations reachable from root. Reject unknown properties and excess nodes with ValueError. Return IDs in depth-first postorder. Progressive protocol updates may temporarily contain missing references; this exercise deliberately validates complete checkpoints.