Add save migration fixtures and failure-mode coverage

src/autoload/save.gd

@@ -14,8 +14,13 @@ func _load_json(file_name: String) -> Variant:
 	var text := FileAccess.get_file_as_string(path)
 	if text.is_empty():
 		return null
-	var parsed = JSON.parse_string(text)
-	return parsed
+	# JSON.parse_string logs an engine error on malformed player files. Use the
+	# instance parser so a corrupt save silently falls back to defaults instead
+	# of looking like a crash in the console.
+	var json := JSON.new()
+	if json.parse(text) != OK:
+		return null
+	return json.data
 
 func _save_json(file_name: String, data: Variant) -> Error:
 	var path := "user://" + file_name