Costco has quietly become one of the best-kept secrets in Halloween shopping. While home haunters obsess over Home Depot and Spirit, the warehouse club drops a rotating set of large animatronics at aggressive prices — and because stock is limited and the Costco model is built on limited quantities and rotating buys, the assortment changes without warning. If this year is anything like the ones before, the best items will sell out long before October 31. That warning — from Be