Function mark_jump 

pub fn mark_jump(
    marks: &HashMap<char, (usize, usize)>,
    name: char,
    current_file_index: usize,
    previous_position: &mut Option<(usize, usize)>,
    current_top: usize,
) -> Option<MarkTarget>

Jump to mark name. Returns a MarkTarget describing the destination, or None if the mark is unknown / name is invalid. On a successful jump, records (current_file_index, current_top) into previous_position.

Note: clamping to line_count is the caller’s responsibility, since the destination file’s line_count isn’t known until after a potential file switch.