Einstein's static universe failed because no mechanism existed to produce cosmological redshift without metric expansion. We propose that mechanism: the speed of light c decreasing over cosmic time as the causal propagation rate of a discrete Planck-volume spacetime network, governed by N × ℓ_P³ = constant. This framework produces two classes of physical quantity: those exactly c-independent (hydrogen binding energy, fine structure constant, atomic spectra) and those depending directly on c(t) (redshift, time dilation, Planck length). We adopt the convention ℏ × c = constant — preserving the fine structure constant α and satisfying atomic clock bounds — and derive the normalisation parameter t_0 = λ/H_0 = 3.69 Gyr, which is explicitly distinct from the LCDM expansion age of 13.8 Gyr. Three exact results follow: (1) a VSL redshift formula 1+z = c(t_emit)/c(t_obs) requiring no metric expansion; (2) supernova time dilation of exactly (1+z), definitively distinguishing the framework from Tired Light; (3) preservation of the hydrogen recombination energy scale at z ~ 1100, where c was approximately seven times its present value. The CMB first acoustic peak position ℓ_1 ~ 220 places a stringent constraint: the VSL parameter λ must be near-zero at z > 0.4, rising to ~0.277 only in the late universe. The derivation of this evolving-λ(t) function from first principles is identified as the framework's central open problem. Three falsifiable predictions are made: primordial gravitational wave suppression r ≈ 0 (testable by LiteBIRD ~2028); fine-structure constant drift only at z < 0.4 at rate dα/α/dt ~ 3.6×10⁻¹⁴ yr⁻¹ (testable by ELT/ANDES ~2030); and excess mature galaxy structure at z > 10, consistent with current JWST observations. Einstein's abandonment of the static universe may have been premature: the mechanism he lacked is a physically motivated variable speed of light grounded in discrete Planck-scale spacetime. This paper is a focused standalone presentation of the core cosmological framework. A seven-paper companion series covering quantum mechanics, antimatter, quantum gravity, QFT, quantum computing, and dark matter is published separately (see Related Works).