Zitterbewegung as Oscillation Across the Cosmological Time-Reversal Boundary

We prove that the Zitterbewegung of a free Dirac fermion — the rapid trembling at angular frequency ω = 2mc²/ℏ identified by Schrödinger in 1930 — is the physical signature of oscillation across the cosmological time-reversal boundary. In a T-symmetric cosmology the negative-energy solutions of the Dirac equation, identified via the Feynman–Stueckelberg correspondence as the particle propagating on the opposite side of the t = 0 boundary, interfere with the positive-energy component to produce the observed trembling. Five structural consequences are proven as theorems: both fermion helicities are observed because the oscillation samples both sides within every Compton cycle; the 4π periodicity of spin-½ arises from the two-sheeted topology; the Majorana condition is forced by boundary geometry; the neutrino is the most delocalised Standard Model fermion; and the massless lightest neutrino m_{ν₁} = 0 is predicted by Bott periodicity of the Clifford algebra tower, falsifiable by KATRIN and Euclid. The Weyl vector Casimir of Gr(2,4) gives ⟨ρ_G, ρ_G⟩ = 5 exactly. We conjecture Ω_DM/Ω_b = 5, consistent with Planck 2018 (5.36 ± 0.10). The shadow symmetry Δ ↔ 2 − Δ encodes the same oscillation in spectral space, linking the Zitterbewegung to celestial holography.