Spacefield Transformation I: A Geometric Closure Framework for Cosmological Mass, Confinement Scale, and Expansion Dynamics

Within the minimal SFT closure framework, the global mass-curvature configuration is constrained by a compactness-saturation condition and a projected mass-area partition. The dimensionless closure structure determines the relative confinement geometry, while the absolute cosmological scale is specified by the baseline confinement normalization eta_min = 2/9 kg m^-2. Conditional on this minimal SFT normalization, the framework yields M_m approximately 9.740 × 10^53 kg and R_c approximately 1.4466 × 10^27 m. These quantities define the baseline mass and confinement scales used for the subsequent reconstruction of the late-time cosmological background. Observed cosmological energy fractions are then used to reconstruct the present cosmological background associated with the closure state. The framework yields a Hubble scale and dark-energy density consistent with current observational constraints, together with the transport law rho_DE(a) = rho_DE,0 a^[3(1-alpha)], which reproduces Lambda-CDM in the limit alpha = 1 while allowing small observationally testable deviations for alpha not equal to 1. For representative departures |alpha - 1| of approximately 10^-2, the parametrised background produces sub-percent to percent-level modifications to late-time expansion and cosmological distance observables relative to Lambda-CDM. These calculations establish conditional observational signatures through which the proposed boundary-scaling sector may be constrained, including independent tests with electromagnetic distances and gravitational-wave standard sirens.