Function toggle_entries_at 

pub fn toggle_entries_at(
    entries: &mut Vec<FileTreeEntry>,
    index: usize,
) -> Result<(), Error>

Toggle expansion of the directory entry at index (1-based within entries). Children of newly-expanded directories are inserted right after the directory’s row at depth + 1; closing a directory removes every entry whose depth is strictly greater than the closing one until depth drops back. No-op for non-directory rows.

Pure function over the entry list – no struct state, no rope re-render. The App-side caller composes this with render_to_buffer and a FileTreeEntries buffer-local write to atomically refresh both halves of the file-tree representation.