Your team stores IoT sensor readings in a BigQuery table `project.sensors.readings` with columns `sensor_id` (STRING), `reading_time` (TIMESTAMP), and `temperature` (FLOAT64). You need to create a new table that adds a column `avg_temp_7d` containing, for each row, the average temperature of that sensor over the preceding 7 days (including the current row). Which SQL feature should you use to compute this efficiently?