An Analytic Discriminant Boundary for the Koide-Locus Jordan Cubic: q_max = (sqrt(2)-1)/54

We give a short, self-contained algebraic proof that the normalized cubic norm on the Koide locus has the discriminant boundary q\_max = (sqrt(2)-1)/54. The calculation uses only the Koide condition s(A)=t(A)^2/6 for a rank-three Jordan element and the elementary discriminant formula for a cubic polynomial. No experimental mass value, fitted parameter, or physical input enters the proof. The note is deliberately narrow in scope: it records one closed-form algebraic boundary value and separates that claim from the broader existing literature relating the exceptional Jordan algebra, charged-fermion mass ratios, and Koide-type relations.