Add missing stickman placeholder and stage scene I/O functionality

- Implemented `MissingStickmanPlaceholder` class to visually indicate missing stickman assets in the scene.
- Created `StageSceneIO` class for handling JSON serialization and deserialization of stage documents, including encoding and decoding of actions and rules.
- Added tests for vector handling, rule encoding/decoding, safe file writing, root validation, and handling of missing assets.
- Introduced a new stickman asset file `bad.stk` for testing purposes.
- Enhanced test coverage for phase 5 file features, ensuring robust handling of scene data and missing assets.

scripts/stage_spawner.gd

@@ -127,18 +127,60 @@ func spawn(id: String, world_position: Vector2) -> Node2D:
 	var pos := world_position + offset
 	match String(entry.get("kind", "")):
 		"terrain":
-			return _spawn_terrain(entry, pos)
+			return spawn_terrain_asset(id, pos)
 		"prop":
-			return _spawn_prop(entry, pos)
+			return spawn_prop_asset(selected_prop_id, pos)
 		"stickman":
-			return _spawn_stickman(pos)
+			return spawn_stickman_asset(selected_stickman_path, pos)
 		"area":
-			return _spawn_area(pos)
+			return spawn_area(pos)
 		_:
 			push_warning("StageSpawner: unknown spawn kind '%s'." % entry.get("kind", ""))
 			return null
 
 
+## Load-safe factories take real object origins and never apply palette offsets.
+func spawn_terrain_asset(asset_id: String, object_position: Vector2) -> TerrainBlock:
+	var entry := _find_entry(asset_id)
+	if entry.is_empty() or String(entry.get("kind", "")) != "terrain":
+		push_warning("StageSpawner: unknown terrain '%s'." % asset_id)
+		return null
+	return _spawn_terrain(entry, object_position)
+
+
+func spawn_prop_asset(asset_id: String, object_position: Vector2) -> PropBlock:
+	var template: Dictionary = PROP_LIBRARY.get_entry(asset_id)
+	if template.is_empty():
+		push_warning("StageSpawner: unknown prop '%s'." % asset_id)
+		return null
+	var prop: PropBlock = PROP_UTILS.spawn_prop(_world, object_position, template["payload"], int(template["material_preset"]), Vector2.ZERO)
+	prop.spawn_id = asset_id
+	return prop
+
+
+func spawn_stickman_asset(stk_path: String, root_position: Vector2) -> StickmanRig:
+	var normalized := stk_path
+	var data: Dictionary = _stickman_cache.get(normalized, {})
+	if data.is_empty():
+		data = STICKMAN_FACTORY.load_stk(normalized)
+		_stickman_cache[normalized] = data
+	if data.is_empty():
+		push_warning("StageSpawner: no stickman data for '%s'." % normalized)
+		return null
+	var rig: StickmanRig = STICKMAN_FACTORY.spawn_from_data(data)
+	if rig == null:
+		push_warning("StageSpawner: failed to spawn stickman.")
+		return null
+	rig.position = root_position
+	rig.source_stk_path = normalized
+	_world.add_child(rig)
+	return rig
+
+
+func spawn_area(object_position: Vector2) -> TriggerArea:
+	return _spawn_area(object_position) as TriggerArea
+
+
 ## World-space AABB for a spawnable node: union of the Polygon2D child's world
 ## points; for a stickman rig, union of its mounted `Body/*` shape geometry; a
 ## point rect otherwise.
@@ -257,11 +299,7 @@ func _spawn_terrain(entry: Dictionary, world_position: Vector2) -> TerrainBlock:
 
 
 func _spawn_prop(entry: Dictionary, world_position: Vector2) -> PropBlock:
-	var t: Dictionary = PROP_LIBRARY.get_entry(selected_prop_id)
-	if t.is_empty():
-		push_warning("StageSpawner: unknown selected prop '%s'." % selected_prop_id)
-		return null
-	return PROP_UTILS.spawn_prop(_world, world_position, t["payload"], int(t["material_preset"]), Vector2.ZERO)
+	return spawn_prop_asset(String(entry.get("id", selected_prop_id)), world_position)
 
 
 func _spawn_area(world_position: Vector2) -> Node2D:
@@ -273,20 +311,7 @@ func _spawn_area(world_position: Vector2) -> Node2D:
 
 
 func _spawn_stickman(world_position: Vector2) -> StickmanRig:
-	var data: Dictionary = _stickman_cache.get(selected_stickman_path, {})
-	if data.is_empty():
-		data = STICKMAN_FACTORY.load_stk(selected_stickman_path)
-		_stickman_cache[selected_stickman_path] = data
-	if data.is_empty():
-		push_warning("StageSpawner: no stickman data for '%s'." % selected_stickman_path)
-		return null
-	var rig: StickmanRig = STICKMAN_FACTORY.spawn_from_data(data)
-	if rig == null:
-		push_warning("StageSpawner: failed to spawn stickman.")
-		return null
-	rig.position = world_position
-	_world.add_child(rig)
-	return rig
+	return spawn_stickman_asset(selected_stickman_path, world_position)
 
 
 static func _points_center(pts: PackedVector2Array) -> Vector2: