Function rolling_sum_i64 

pub fn rolling_sum_i64(data: &[i64], window: usize) -> Vec<Option<i64>>

Rolling sum over i64 data with O(n) sliding window.

Positions before the window is full are set to None (caller maps to NaN or whatever sentinel is appropriate). Uses wrapping arithmetic.

Returns a Vec of length data.len(). Elements 0..window-1 are None.