completeMultiFileDownload captured an index via firstIndex, then awaited a
detached file-size calculation. Another concurrently-completing download could
mutate activeDownloads during that suspension, leaving the index stale and
crashing in Array.remove(at:) with an out-of-bounds index. Remove the download
by identity instead.
Fixes a SIGTRAP seen in TestFlight build 261 (5 reports).
ec5ac944c4