Neumann T = V/(1 − V·G₀); 3rd-order Taylor T ≈ V + V²·G₀ + V³·G₀²; EXAFS sinusoid

Sympy-exact symbolic witness of the XANES Neumann-series T-matrix and the single-scattering EXAFS envelope. Setup: V, G₀, N_path, S₀, k, R, f_k, δ sp-symbols. Closed-form Neumann resolvent: T(V, G₀) = V/(1 − V·G₀); sympy canonical…