Structural Stability and Degeneracy Lifting of the Neutrino Heavy Sector

Neutrino carrier ARC and HNL prediction 17/18 This paper studies the structural robustness of the source-qualified neutrino heavy sector and determines which features persist under controlled perturbations and which depend on exact symmetry. The preceding PCR papers establish a branch-stable one-carrier neutrino structure with three nondegenerate light sectors, one rank-three heavy sector, the active-deficit/heavy-support relation, and a target-qualified experimental prediction framework. PCR‑7.5 tests the stability of these results under heavy-mass splitting, coupling anisotropy, response deformation, and related source-preserving perturbations. The analysis distinguishes several logically different structural properties: one-carrier identity; nonzero heavy support; rank-three heavy-sector structure; active-sector closure; exact heavy degeneracy; flavor universality; and vanishing heavy charged-lepton-flavour violation. These properties do not all have the same robustness status. Carrier identity remains stable on certified trace-transport cells. The active-deficit/heavy-support closure relation is exact on the block-isometry class, and rank-three heavy support persists within a positive perturbation radius. By contrast, exact heavy degeneracy and exact flavor universality are symmetry-protected features with zero unrestricted symmetry-breaking radius: arbitrarily small traceless mass perturbations can split the heavy spectrum, while arbitrarily small anisotropic coupling perturbations can break flavor universality without necessarily destroying rank. The paper develops a source-first perturbation and response calculus that preserves the distinction between physical carrier structure, spectral-sector labels, detector response, and presentation-dependent coordinates. Heavy-sector mass eigenvector labels, basis choices, and matrix representations do not by themselves create new carrier identities. A resolved heavy split therefore tests the exact symmetry-protected degeneracy stratum rather than automatically falsifying the one-carrier theorem. The analysis also retains the experimentally motivated prompt-heavy-neutral-lepton domain, including the source-qualified 28–30 GeV search window and the \(c\tau_A=0.2\) mm response-grid point. These coordinates are treated as a justified scouting benchmark with positive applicability margins, not as an exact physical pole-mass or proper-lifetime prediction. The paper provides a feature-specific falsification map. A loss of rank, failure of active-deficit/heavy-support closure, nonzero qualified heavy CLFV, or loss of carrier identity places pressure on different structural layers and carries different implications. This prevents a symmetry refinement failure from being misidentified as a carrier failure, while preserving genuine tests of the source-qualified heavy-sector construction. The conclusion is deliberately bounded. The paper establishes robustness only on the declared perturbation and trace-transport domains. It does not claim unrestricted stability, exact physical degeneracy under arbitrary perturbations, a unique microscopic origin, a numerical complex pole, present-facility sensitivity, or empirical confirmation of the heavy sector.