Function map_index_after_move 

pub fn map_index_after_move(
    idx: usize,
    from: usize,
    to: usize,
    count: usize,
) -> usize

Map an index through a DataChange::ItemsMoved { from, to, count }.

Mirrors ListModel::move_item: the contiguous block from..from+count is removed, then reinserted so its first item lands at to (a post-removal index). Returns where idx ends up after the move. Used by index-based state (selection, checked-set) to follow items across a reorder.