Function swar_add_body 

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

The PACKED SWAR add — one datapath op that blocks carry at every lane boundary: ((a & M) + (b & M)) ^ ((a ^ b) & ~M), where M clears each lane’s top bit. This is what the SIMD tile emits; simd_tile_lock certifies it equals lanewise_add_body.