The period-doubling route to chaos, with its universal Feigenbaum constants, governs a broad class of nonlinear dynamical systems. The Universal Cascade Law (UCL) formalizes this universality precisely: any nonlinear coupled system satisfying analytic dissipative boundedness with a compact absorbing set (C₁), a non-degenerate extremum of specified order in the return map (C₂), and an infinite accumulating sequence of transversal period-doubling bifurcations (C₃) exhibits cascade structure governed by universal constants determined by the critical order alone. We prove that Mandelbrot's iteration z → z² + c is the unique canonical form of the minimum topology class satisfying C₁, C₂, C₃. Linear maps (topology class z = 1) possess no critical point and fail C₂; the quadratic class (z = 2) is therefore the minimum qualifying topology by necessity — the ground state of the cascade architecture. Every degree-2 complex polynomial is conjugate to z → z² + c, making Mandelbrot's equation the canonical form of this ground state, not merely a representative. The Feigenbaum constant δ = 4.6692... governing the z = 2 class follows from Lanford's uniqueness theorem applied to the renormalization fixed-point equation; the classification is proved necessary. Higher topology classes (z = 3, 4, 6, ...) carry distinct Feigenbaum constants and govern cascade structure in higher-order critical systems. We establish that Mandelbrot's equation describes middle-scale three-dimensional dynamics precisely because that scale domain corresponds to the z = 2 ground state; quantum systems (infinite-dimensional Hilbert space) and gravitational systems (tensor fields in 4D spacetime) require higher topology classes that z → z² + c cannot describe. The relationship is structurally identical to that between Newtonian gravity and general relativity: a correct special case within a domain, derivable from the general theory, not fundamental to it.