Electromagnetism Inherits the Light Cone: the Speed of Light as the Unique Constant of the Absolute Frame Interface

In the Absolute Frame Theory (AFT), the observable four-dimensional manifold M is continuously embedded in an N-dimensional Euclidean substratum A. We argue that the speed of light c is the unique dimensionful constant intrinsic to the M--A interface: it is the single parameter of the causal structure that emerges when the positive-definite substratum metric is analytically continued to the Lorentzian signature of M. The same constant appears in three manners --- the elastic-wave speed of the substratum, the opening of the light cone of the induced metric gμν, and the signal velocity of the M--A information channel --- which coincide because the embedding is at once an elastic membrane, a metric pull-back, and an information channel. Electromagnetism does not define c: the constants μ₀,ε₀ are emergent impedance properties of M, Maxwell's relation c=1/√μ₀ε₀ is the electromagnetic face of the same invariant, and the photon travels at c because the unbroken U(1)ₑₘ leaves it massless --- it rides the cone rather than setting it. We separate what is derived (the form of c and the maximal-stiffness identity YA=ρA), what is empirically anchored (the equality of the gravitational-wave and electromagnetic speeds; the Michelson--Morley null result), and what fixes the substratum scale (the density ρA, pinned by its companion role as the cosmological constant). The reading inverts the historical logic that made c ``the speed of light'': c is the geometry, and light is one of its tenants.