The U-Cell Model as a Foundation for Quantum Field Theory: ħ from Minimal Cell Action, Physical UV Cutoff, and the Three Fundamental Constants

The Feynman path integral rests on the existence of a continuous spacetime — a prerequisite standard QFT takes as given. The U-Cell Model (UCM) identifies this continuum as an effective macroscopic approximation valid for E ≪ E_Λ = ħc/a, where a = ℓPlanck is the physical lattice spacing. UV divergences of standard QFT arise precisely where this approximation breaks down. Before the continuum approximation, the path integral measure is a well-defined product over countably many lattice sites; no regularisation is needed. The reduced Planck constant is derived from the minimal action of a single U-cell Scell ~ ρ₀a⁴c, with the numerical factor α = 12π determined by the cubic lattice geometry (6 nearest neighbours × 2π phase space each). The Planck length ℓPlanck = a then follows as a consistency consequence, not a circular input. All three fundamental constants follow from three substrate parameters (ρ₀, a, κc): c = √ (κc/ρ₀), G = c²/ (12π ρ₀ a²), ħ = 12π ρ₀ a⁴ c. The Planck mass equals 12π times the cell mass. The vacuum zero-point energy does not curve spacetime locally (no substrate flow gradient for a homogeneous source), while global expansion is driven by substrate creation in voids (wₑff → -1, Part D) — cleanly separating the cosmological constant problem from the dark energy mechanism. Three open problems are identified: derivation of ℓPlanck = a from first principles, canonical commutation relations from substrate dynamics, and loop corrections to vacuum energy coupling.