Structural Reconstruction of the Neutrino Sector from Neutral-Parent Closure: PMNS Mixing, Rank-Two Mass, and an Exactly Massless Eigenstate

Three-flavor oscillation theory parametrizes neutrino mixing and mass-squared splittings but does not explain the observed PMNS orientation or the absolute mass spectrum. This paper develops a structural reconstruction of both within a proposed neutral-parent carrier-closure framework. The construction distinguishes a premetric selection layer from the effective field theory that governs dynamics after spacetime read-out. At the premetric layer, automorphism-inequivalent admissible branches receive equal primitive weight; their multiplicities select a tribimaximal leading frame, while previously derived charged-lepton root ratios determine two controlled deformations. For the mass sector, a linear exposure map sends three neutral continuation modes to a two-dimensional transverse closure quotient. Under explicit assumptions of strict factorization, protection covariance, and transverse saturation, the associated positive mass-squared operator has rank two and one exactly massless eigenstate. A minimal two-seam complex then fixes the two nonzero eigenvalues. The resulting normal-ordering spectrum is m₁ = 0, m₂ = 8.686 meV, and m₃ = 50.092 meV, with a mass sum of 58.778 meV and mass-squared splittings close to current global-fit values. No measured neutrino angle, phase, splitting, or absolute mass is inserted into the structural formulas; the measured electron mass only converts dimensionless ratios to electronvolts. The construction supplies spectral boundary data that a post-read-out Lagrangian theory must reproduce rather than replacing standard quantum field theory. Present numerical agreement is retrospective; the frozen rank, spectrum, and mixing correlations provide prospective tests.