How often it ran out

Two different problems look identical in a daily report. A product missing for one long stretch is a supply failure at the warehouse; one that keeps blinking out and coming back is a reorder point set too low at the store. Telling them apart needs a count of stretches, not of empty days.

For every store and product, return the store_id, the sku, and how many separate runs of out-of-stock days it had as stockout_spells. Leave out any store and product that was never out of stock.