Untwisting the double copy: the zeroth copy as an optical seed

We present a historical optical foundation for stationary vacuum Kerr–Schild spacetimes on a flat background and interpret it in modern double-copy language. In this setting, a complex optical seed $\rho =-\theta -i\omega$, built from the expansion and signed twist of the Kerr–Schild congruence, is harmonic, while its inverse obeys an eikonal equation and reconstructs the congruence algebraically. Thus the local stationary geometry is organized by a single complex seed. In the overlap of the stationary Kerr–Schild and Petrov type–D Weyl double-copy framework, this seed furnishes a normalized representative of the zeroth-copy data, while its real part yields the Kerr–Schild profile and its gradient generates the single-copy gauge-field strength. The construction provides, without recourse to twistor methods, a spacetime realization of how a single complex seed builds the congruence, organizes the associated spacetime and gauge fields, and encodes the geometric content of the zeroth copy.