A sister paper to "Geometric Origin of CP Violation" v15 (DOI: 10. 5281/zenodo. 19545893). The companion paper reports a CKM matrix reconstruction with zero free parameters that reproduces all eleven CKM observables within 0. 52 sigma, built through a test-driven development (TDD) program. A single empirical input remains: the Koide phase thetaK, which parameterizes the quark mass spectrum. This paper attempts a pure-geometry reconstruction of thetaK itself, using only geometric constants already established in the companion paper (Fano rapidities, hierarchy r = pi/8, Fano point count NFano = 7, generation integers ell = 1, 2, 3), and proposes: thetaKᵤp = r * muₖ = (pi/8) (ln2/ln3) ~ 0. 24784 (empirical 0. 247, 0. 34% difference) Delta thetaK = 1/ (2 Sigma ell) = 1/12 ~ 0. 08333 (empirical 0. 083, 0. 40% difference) thetaKdown = thetaKᵤp - Delta thetaK ~ 0. 16451 (back-solved 0. 164, 0. 31% difference) The denominator 2 Sigma ell = 12 coincides with the numerator of the exact k-edge EM coefficient 12/7 = 2 Sigma ell / NFano derived at Step 33 of the companion paper. This multiple appearance of the Fano global algebraic quantity 2 Sigma ell in two logically independent constructions (EM phase correction and Koide angle difference) is, within the present framework, the strongest structural argument against accidental coincidence. This paper is NOT a first-principles proof. The mapping between the Koide harmonic sphere and the Fano rapidity simplex is not constructed. The proposal is a structural candidate audited against four criteria (numerical precision, uniqueness, multiple appearance, derivability), with honest C labels on remaining open gaps. Published as a separate sister paper to preserve the methodological distinction: the companion paper achieves precision through TDD against PDG measurements; the present paper achieves structural consistency through pure-geometry reconstruction. Keywords: CKM matrix, Koide mass formula, Fano plane, Kerr geometry, quark masses, pure geometric reconstruction.
Kuniyuki Hayashi (Mon,) studied this question.