It is a operational reality of astrophysics that observers do not measure continuous 3D bulk volumes, but rather the 2D decoupling surfaces where radiation escapes. In this Letter, the operational requirement of radiative decoupling is formalized as a geometric boundary condition. By coupling measurable radiative observables with Newtonian surface gravity, a dimensionless boundary invariant, 𝒳, is defined. Evaluated analytically under standard macroscopic closures, 𝒳 reduces to an exact mass-independent identity. Observationally, using strictly independent solar measurements and a benchmark kinematic sample of 190 detached eclipsing binaries, stellar photospheres are found to lie below the theoretical blackbody phase-space capacity 𝒳₀ = π³/15 ≃ 2.0671. Projecting this strictly 2D surface normalization to the cosmological horizon boundary yields the unified master identity ΛR_H² = π³/15, linking the cosmological constant to the horizon area. This geometric relation provides a parameter-free prediction for the dark energy density fraction Ω_Λ = π³/45 ≃ 0.6890. The canonical ~10¹²⁰ cosmological constant discrepancy is reframed not as an energy divergence, but as the dimensional area-scaling ratio (A_H/l_P²) required to map the macroscopic boundary to the microscopic Planck scale. Assuming spatial flatness, the matter fraction is geometrically constrained to Ω_m = 1 − π³/45 ≃ 0.3110. Finally, modeling the spatial transition from a continuous primordial fluid to a discrete late-time void network via the 3D optimal kissing number (k = 12) derives a kinematic volume scaling of H₀^local = H₀^CMB(13/12) ≃ 73.01 km s⁻¹ Mpc⁻¹. The resulting framework introduces exactly zero free parameters, providing a geometric interpretation of prominent cosmological tensions as rigid topological imperatives of spatial boundaries.