feat: add Phase 20a stage identities and overlap selection

tests/test_content_paths.gd

@@ -57,8 +57,13 @@ func _test_moved_stage_file_paths() -> void:
 func _test_legacy_stage_round_trip() -> void:
 	_cleanup()
 	var legacy_data: Dictionary = {
-		"version": STAGE_SCENE_IO.FILE_VERSION,
+		# Keep this fixture at the pre-identity schema so the path test also
+		# exercises the existing legacy-stage migration into the current format.
+		"version": "1.7",
 		"scene_name": "Legacy Asset Paths",
+		"sky_id": "sky_neutral",
+		"background_kind": "sky",
+		"plain_color": "#F5EFE0",
 		"camera": {"position": {"x": 0.0, "y": -400.0}, "zoom": 0.5},
 		"objects": [
 			{