We give a self-contained report on a finite residual-shell reduction program for the Erdős–Straus equation. The hard congruence class is organized at the modulusM₂₃ = 840 · 11 · 19 · 23 = 4,037,880over the six square classesS₈₄₀ = {1, 289, 361, 169, 121, 529} = ⟨289⟩ ≅ C₆.The paper proves the finite elementary certificate criterion, the initial restricted reduction to 8554 residue classes, the full prime-shell reduction to 5828, the first composite reduction to 5720, the corrected divisor-visible saturation to 4951, and the lifted non-visible smooth-shell framework. The all-M₂₃-smooth elementary closure leaves exactly2970 = 6 · 5 · 9 · 11base classes, with sector vector(495, 495, 495, 495, 495, 495).This terminal core is the productC₆ × Q₁₁ × Q₁₉ × Q₂₃,where Q_q = (𝔽_q^×)². Consequently every nontrivial C₆-Fourier mode vanishes and the obstruction is exactly the C₆-trivial idempotent. We then prove the formal equivalence between this sixfold central-trivial arithmetic fiber and the central Z₆ quotient structure of the Standard Model gauge group[SU(3) × SU(2) × U(1)] / Z₆.The comparison is theorem-level: the Standard Model conditionq ≡ 3z₂ − 2z₃ (mod 6)is the same Chinese-remainder reconstruction law as the residual relationk ≡ 3(k mod 2) − 2(k mod 3) (mod 6)under the identifications r_k mod 5 = (−1)ᵏ and r_k mod 7 = 2ᵏ. We close by recording the finite determinant, residue-zeta, and pure H⁰ trace forms of the obstruction, and we isolate the first necessary escape from the smooth elementary envelope: external-prime lifted congruence fibers or non-elementary divisor-box certificates.