Function lanewise_add_body 

pub fn lanewise_add_body(ty: LaneTy) -> (Vec<FabricOp>, usize)

The LANE-WISE reference for packed add — the SPEC: each lane independently computes (a_j + b_j) mod 2^w, expressed as an AluOp residual (shift the lane down, add, mask to width, shift back, OR the lanes). This is “N independent scalar adds” made concrete.