Fix the replay of a search box race

A search box fires one request per keystroke and shows whichever response lands last, so a slow response for "ca" can replace the results for "cat". The fix is an effect cleanup: when the query changes, the effect disowns the request it started, and only the newest request may write.

An assistant wrote displayed_query(issued_queries, arrival_order, ignore_stale). It is in the editor. It has a bug. Find it and fix it.

Input and output:

• issued_queries: the query of each request, in the order the requests were sent. Request i was sent for issued_queries[i].
• arrival_order: request indices, in the order their responses came back. A request whose response never came back is missing. An index can repeat or fall outside the list.
• ignore_stale: true with the cleanup guard, false for the shipped code with no guard.
• Return the query whose results are on screen after the last arrival, or "" when nothing has been written.

Rules the tests check:

• No requests returns "".
• Ignore any index below 0 or not below the number of requests.
• Without the guard: every response writes. Return the query of the last valid arrival, or "" when there is none.
• With the guard: only the newest request, the last one sent, may write. A response from any earlier request never writes, whenever it arrives. Return the newest query if its index appears anywhere in arrival_order, otherwise "".