Physics currently treats the vacuum catastrophe, dark energy, and the Hubble tension as distinct crises. We propose they are symptoms of a single dimensional category error: modeling macroscopic expansion as a 3D bulk fluid rather than a 2D boundary tension. Evaluating equilibrium at radiative decoupling surfaces causes explicit mass dependence to cancel, yielding a purely geometric invariant: pi^3/15 ≈ 2.0671. Projecting this limit to the cosmic horizon gives the observed dark energy fraction, Omega_Lambda = pi^3/45 ≈ 0.6890, without tunable parameters. This predicts the acceleration onset, relaxes early dark matter requirements, and reframes the Hubble tension as geometry. Area-governs-volume is established in black holes and quantum boundary potentials; the universe may follow the same rule.