Density Field Dynamics (DFD) is a scalar refractive-index theory of gravity defined by the postulate that spacetime is flat but permeated by a scalar field ψ(x,t) establishing an optical refractive index n = exp(ψ). Light propagates according to the eikonal of the optical metric ds̃² = −c²dt²/n² + dx², while matter responds to the effective potential Φ = −c²ψ/2. This framework has an optical scalar sector ψ that governs clock rates, refraction, and quasi-static dynamics, together with a transverse-traceless radiative sector h^TT_ij for gravitational waves. It reproduces all classic tests of general relativity in the weak-field limit (γ = β = 1, all PPN parameters matching GR), gravitational waves at speed c with two tensor polarizations, and MOND-like phenomenology at galactic scales through a nonlinear crossover function μ(x) = x/(1+x) and scale a* = 2√α cH₀, both derived from S³ topology (Appendix N). A dedicated model-independent SPARC shape analysis finds n_opt = 1.15 ± 0.12 (95% CI [1.00, 1.50]) in the family μ_n(x) = x/(1+x^n)^{1/n}, with DFD's n = 1 inside the confidence region and Standard MOND's n = 2 strongly disfavored. This paper presents DFD as a unified framework: (1) Fine-structure constant: α⁻¹ = 137.036 from the microsector spectral action on ℂP² × S³ with Toeplitz truncation at k_max = 60. The derivation is convention-locked: a forced binary fork between regular-module and fermion-rep microsectors is resolved by a no-hidden-knobs policy, with the surviving branch matching experiment at sub-ppm level. Verified by lattice Monte Carlo (L6–L16; 9/10 at L16 with p 7 yr) Distance duality e^{Δψ} factor present Etherington reciprocity holds exactly; DDR = consistency check, not estimator (v3.1 erratum) What's New in v3.0 Result v2.0 Status v3.0 Status G = diag(2/3, 1, 1) Input parameter Derived (Theorem K.4, primed trace) Fermion masses 1.9% mean error 1.42% mean error (explicit A_f) sin²θ_W = 3/13 Not present Derived (trace normalization) α_s(M_Z) = 0.1187 Not present Derived (Λ_QCD scaling) Neutrino spectrum Partial Fully closed (Δm² match NuFIT, p=0.99) P(k) confrontation Not addressed β closed (ΛCDM match at O(10⁻⁵)) K_d, K_u kernels Conjectured Derived (Lemma K.2, symmetry) Lattice verification L4–L12 L6–L16 (9/10 at L16, p < 0.01) What's New in v2.0 Result v1.0 Status v2.0 Status μ(x) = x/(1+x) Phenomenological fit Theorem (S³ composition) a* = 2√α cH₀ Proposed relation Theorem (stationarity) Dust branch (w→0) Not addressed Theorem + no-go lemma Strong CP (θ̄=0) Tree-level argument All-loops theorem (dim-8) G–H₀ = α⁵⁷ Proposed coincidence Dictionary-closed Screen-closure Not present Overdetermined falsifier k_α, M_R Not derived Derived (Appendix P)