feat: add Phase 20a stage identities and overlap selection

scripts/props/prop_block.gd

@@ -31,6 +31,7 @@ var spawn_id: String = ""
 ## Stable identity of this placed prop in one Sandbox Stage scene. Unlike
 ## spawn_id, this value is unique per instance and survives scene save/load.
 var stage_object_id: String = ""
+var stage_name: String = ""
 var interaction_anchors: Dictionary = {}
 var interaction_reserved_by: int = 0
 var interaction_owner_id: int = 0