ocean_evolve

Advances one cascade's static spectrum to the current time — h(k,t) = h0(k) e^{iwt} + conj(h0(-k)) e^{-iwt} at the deep-water dispersion w = sqrt(g|k|) — and emits the two complex fields the inverse FFT consumes. The frequency is quantised to a multiple of 2*pi / loop_period so the whole sea state repeats seamlessly instead of drifting as elapsed time grows. Field A packs the height and X-displacement spectra into one complex field (H + i Dx), which the Hermitian symmetry of both lets a single inverse transform separate again — two FFTs per cascade instead of three.