We construct a pre-Niemeier datum from the residual divisor calculus for the Erdős–Straus equation. The fixed-shell divisor condition is first normalized by the scale N = pa, converting the raw congruence d ≡ -N (mod R) into the invariant target -1 in the finite torus (ℤ/Rℤ)×. The resulting shell is a bounded signed-exponent support problem. After quotienting the divisor involution, the three raw p-origins collapse to two normalized channels, with targets -1 and -p⁻¹. The modulo-840 sieve supplies a four-fiber support fibration V₈₄₀ → C₂² with cyclic six-point fibers. The fiber augmentation lattices and the D₄ base form a rank-24 equi-Coxeter root frame A₅⁴D₄. We construct an explicit maximal isotropic subgroup of order 72 in the discriminant form of A₅⁴D₄, prove that the corresponding overlattice is even unimodular, and prove that its root system remains exactly A₅⁴D₄. Thus the residual support fibration closes to the Niemeier lattice of root type A₅⁴D₄. The later residual shells become finite conservative matrix field reachability problems on the selected Coxeter component. We also record the E₈ Coxeter-exponent calibration: the cyclic residual fiber maps modulo 30 to the square-exponent sector of E₈.