Cheapest stock items

A grocery app offers replacements when a shopper's item is unavailable. skus holds the candidate product codes, cents holds each candidate's shelf price in cents, in_stock says whether the warehouse can actually pick that candidate, and the three lists are the same length. Return the product codes of the k cheapest candidates the warehouse can pick, cheapest first. Candidates that are not in stock are dropped before anything is chosen, so k counts only pickable ones. Two candidates on the same price are ordered by product code ascending as ordinary strings, compared character by character, so SKU-10 comes before SKU-9. Product codes are distinct. When fewer than k candidates survive the stock filter, return every survivor in that order, and when k is zero or negative return an empty list.