Function binary_search_exact_pos 

pub fn binary_search_exact_pos(
    v: &[Rc<str>],
    iid_to_row: &HashMap<Rc<str>, usize>,
    target_row: usize,
) -> Result<Option<usize>, String>

Performs a binary search on a sorted vec of IIDs to find the exact position where the row == target_row. Assumes the vec is sorted by row numbers. Returns Some(pos) if found, None if not present, or an error if any accessed IID lacks a row mapping.