Function calculate_emphasis_range 

pub fn calculate_emphasis_range(
    line: usize,
    line_content: &str,
    start_pos: usize,
    end_pos: usize,
) -> (usize, usize, usize, usize)

Calculate range for emphasis markers and content

Safety

This function safely handles multi-byte UTF-8 characters by ensuring all string slicing operations occur at valid character boundaries.