Function emit_ntt_butterfly_verilog 

pub fn emit_ntt_butterfly_verilog() -> String

Emit the combinational Montgomery fqmul + one Cooley–Tukey butterfly: inputs a, b (the two coefficients) and zeta; outputs hi = a + t, lo = a - t where t = fqmul(zeta, b) — all wrapping 16-bit, matching ntt_scalar’s inner step.