Migrating from AGNI in DESC

AGNI in DESC is PR #1893, built on the differentiation matrices of PR #1789.

Before, the eigenvalue came from DESC's compute machinery:

data = eq.compute("finite-n lambda3", grid=grid, diffmat=diffmat, gamma=5/3)

Now:

basis = agnimhd.Basis(n_rho, n_theta, n_zeta)
eq_data, diffmat = agnimhd.from_desc(eq, basis)
gamma2 = agnimhd.growth_rate(eq_data, diffmat, agnimhd.AssemblyConfig(gamma=5/3))

The sign is flipped: agnimhd returns gamma^2 = -lambda3, positive when unstable, and its SolverConfig.sigma is minus DESC's sigma (for example 0.1 for DESC's -0.1).

Keyword options and AGNI_* environment variables became fields of AssemblyConfig and SolverConfig. Environment variables are no longer read.

DESC	agnimhd
gamma, incompressible, axisym, coupled_rt	AssemblyConfig
sigma, num_matvecs, eigensolver	SolverConfig
eigensolver="eigsh_callback"	"eigsh"
eigensolver="pcg_deflated" with jd options	"jd", coarse=basis.coarse_level(eq_c, family)
FinitenStability objective	agnimhd.adapters.desc_objective.AgniStability

Gradients: in DESC, AGNI is a compute function of R_lmn, Z_lmn, p_l, i_l, Psi, so jax.grad reaches the parameters directly and ProximalProjection keeps force balance. Here the caller supplies that parameter-to-grid map: jax.grad(growth_rate)(params, diffmat, equilibrium_map=equilibrium_map). jax.grad(growth_rate) on an EquilibriumData raises.

The agnimhd copy was taken from DESC commit f625b0121 (2026-08-18) and has since received DESC's later fixes up to 437ccf2ed. It also fixes a bug DESC still has: fourier_interp_matrix ignored its period argument. A second, on the complex Hermitian (axisym=True) operator, is fixed in both, differently. matfree's Lanczos orthonormalized with Q.T Q instead of Q^H Q and returned a wrong eigenvector with a plausible eigenvalue (+9.713e-02 against a dense -2.660e-03 on one plane of the test case); it was fixed upstream in matfree #288, hence matfree>=0.6.2, while DESC uses a real 2n embedding. And the eigsh callback declared a real output dtype; it now comes from assemble.operator_dtype. On the shipped test case agnimhd reproduces DESC's dense eigenvalue to 7e-10 relative.