The Heptagonal Constraint — Why Vortex Stretching Redistributes

Navier-Stokes blowup requires the strain rate tensor to become axisymmetric — two eigenvalues coalescing while the third diverges. We show that if the strain eigenvalues are constrained to the centred Steinbach polynomial t³ − 7t/3 + 7/27 = 0 — the trace-free form of the minimal polynomial of ρ = 2cos(π/7) — then axisymmetric strain is algebraically inaccessible because the polynomial discriminant Δ = 49 = 7² is nonzero for all strain amplitudes. The centred Steinbach conjugates are the unique trace-free triple from an irreducible cubic with cyclic Galois group (chirality), class number one (rigidity), and minimal discriminant. This single constraint reduces the two free eigenvalue parameters to one (the amplitude A), locks the third invariant to det(e) = −7A³/27, and fixes the Lund-Novikov shape parameter at ξ = −1/(2√7) ≈ −0.189. Four DNS observations are predicted without fitted parameters: shape parameter ξ ≈ −0.189, preferential positivity of the intermediate eigenvalue, eigenvalue ratio λ₂/λ₁ ≈ 0.076, and vorticity alignment with the intermediate eigenvector. The discriminant Δ = 49A⁶ grows with strain amplitude — the opposite of the blowup scenario. Paper 165 of the Bootstrap Universe Programme.