The Structural Law of Observables, O(x) = Obase[1 + ϕ · S(x)], is derived from General Relativity when physical parameters are treated as metric-dependent functionals. The universal structural coupling constant ϕ = 0.51 ± 0.03 is obtained from quantum entanglement spacetime theory (QuEST). The local structural index Slocal = 0.164 is inferred independently from asymptotic decay excess in deep-space Radioisotope Thermoelectric Generator (RTG) data and from the Type Ia supernova mass step. With no free fitting parameters, the law predicts RTG power deficits across six missions with correlation r = 0.80 and correctly predicts the sign reversal between heliospheric regimes. The observed Hubble tension emerges as a quantitative postdiction. A falsifiable prediction is made for late 2026.