Function find_active_window_by_sequence 

fn find_active_window_by_sequence<'a>(
    files: impl Iterator<Item = &'a FileHandle>,
    time_window_size: i64,
) -> Option<i64>

Finds the active window from the file with the highest sequence number.

Flush and compaction outputs inherit the max input sequence, so the max-sequence file always tracks the most recent write: the active window survives the transient state where level 0 is empty right after its files were compacted away. Returns None when no file carries a sequence.

The window key follows the same convention as assign_to_windows (truncate to seconds, then align up), since it is compared against the window keys produced there.