The slots of closed files stay in `files` (empty) for the whole run, and CloseOldFiles visited
all of them on every image: a thousand slots per image for a million-image run. Keep the open
file numbers in an ordered set and walk that; being ordered, the walk ends at the first file
that is not old enough to close.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
leonarski_fandClaude Fable 5.1
6202d36268