Function styled_line 

pub fn styled_line(
    text: &str,
    line_index: usize,
    left_col: usize,
    tab_width: usize,
    selections: &[Selection],
    theme: &ThemeColors,
) -> Line<'static>

Build one rendered line, splitting it into spans wherever the selection state changes.

Spans are cut at grapheme boundaries and styled per run, so a wide character is highlighted as one unit and never half-painted.