We show that an effective cosmological constant arises from two conditions on a bareEinstein–de Sitter cosmology: (i) that the volume-averaged spatial Ricci curvature vanisheson each spatial hypersurface (perturbatively stable in classical GR on T 3), and (ii) that thegravitational entropy satisfies the holographic bound (the Generalized Second Law applied tothe cosmological horizon). In matter domination, the (1 + z)3 scaling of the Hubble rate and thephysical volume cancel identically, rendering the holographically bounded curvature varianceexactly constant — a cosmological constant in the dressed Friedmann equation, with no darkenergy substance introduced. The framework has zero free parameters: all coupling con-stants are derived from information theory (the KL normalization), GR kinematics (the Buchertexpansion-shear decomposition), and the Generalized Second Law.The framework predicts ΩΛ · Ωm = ΩS = constant, dissolving the coincidence problem.Convergent arguments yield the conjecture ΩS = 2/9 (Ωm = 1/3), consistent with DES Year6 (Ωm = 0.333+0.023−0.028). Structure formation saturates the bound at z ∼ 0.5–1.0 on group scales,matching the observed acceleration onset with no parameter tuning. The primary discriminatoris the matter skewness S3 at the saturation scale: HCV predicts S3 ≈ 0.4 (near-Gaussian);standard gravity predicts S3 ≈ 8 (lognormal) — testable with Euclid DR1 weak lensing statistics.Companion papers address the DESI DR2 evolving dark energy signal (Paper II) and galaxy-scale emergent dark matter phenomenology (Paper III).