Function emit_fused_row 

pub fn emit_fused_row(spec: &FusedRowSpec, knobs: &CudaKnobs) -> String

Emit a self-contained CUDA program for a recognized fused row op: one __global__ block per row doing a shared-memory tree reduction over cols, plus a host that fills X deterministically and prints Σ Y. knobs.chip stamps the provenance header (and drives the arch the caller compiles for).