Release 0.8.1

SOLVAX 0.8.1 adds solvax.elliptic, a spectral Fourier–Helmholtz elliptic solve for separable Helmholtz-type problems on a periodic axis and a bounded axis (build_fourier_helmholtz_operator, solve_fourier_helmholtz, FourierHelmholtzOperator). The periodic axis is Fourier-transformed and the remaining per-mode tridiagonal system in the bounded axis is solved in one batched call; the whole inversion is jit/grad/vmap transparent.

This is the \(\nabla_\perp^2\phi=\rho\) inversion used by reduced drift-plane and vorticity models: build the operator once per geometry, reuse it every timestep. See Spectral Fourier–Helmholtz elliptic solve.