BV anchors: j(η=0)=0; linear regime j ≈ j₀·f·η (sp.series O(η²))

Sympy-exact witness of the Butler-Volmer zero-overpotential identity and the linear regime via Taylor expansion. Setup: j = j_0·(exp(αfη) - exp(-(1-α)fη)). sympy computes j(η=0) = j_0·(exp(0) - exp(0)) = 0 exactly (equilibrium identity).…