Function count_visible_rows_between 

pub fn count_visible_rows_between(
    fold_idx: &FoldIndex,
    from_line: u32,
    to_line: u32,
) -> u32

Count how many visible (non-fold-hidden) rows exist between from_line (inclusive) and to_line (exclusive). Closed fold bodies are skipped. Both lines must be visible (not inside a closed fold body) — callers ensure this before calling. Returns the number of display rows spanning the range.