Function evaluate_window_functions_value 

pub fn evaluate_window_functions_value(
    rows: &mut [Vec<Value>],
    column_index: &HashMap<String, usize>,
    specs: &[WindowFuncSpec],
) -> Result<Vec<String>, WindowError>

Evaluate window functions over a Vec<Vec<Value>> result set.

column_index maps column name → position in each row slice. For each spec, one Value is appended to every row. Returns the list of new column names, one per spec in spec order.