feat: add Phase 20a stage identities and overlap selection

scripts/props/missing_prop_placeholder.gd

@@ -4,12 +4,14 @@ extends Node2D
 var original_entry: Dictionary = {}
 var missing_prop_path: String = ""
 var stage_object_id: String = ""
+var stage_name: String = ""
 const MARKER_SIZE := Vector2(180.0, 54.0)
 
 func configure(entry: Dictionary) -> void:
 	original_entry = entry.duplicate(true)
 	missing_prop_path = String(entry.get("asset_id", ""))
 	stage_object_id = String(entry.get("stage_object_id", ""))
+	stage_name = String(entry.get("stage_name", ""))
 	queue_redraw()
 
 func get_area_rect() -> Rect2:

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