Description: This paper proposes a novel interpretation of the Hubble tension—the persistent 8-9% discrepancy between early-universe (CMB/Planck: ~67 km/s/Mpc) and local (Cepheid/supernovae: ~73 km/s/Mpc) measurements of cosmic expansion rate. Rather than treating this tension as a problem requiring resolution through new physics or systematic error correction, the paper argues the discrepancy itself is evidence of fundamental cosmic structure. Synthesizing two frameworks from the Abstract Theories Project—the Harmonic Unification Framework (HUF) and the Equilibrium Pi hypothesis—the paper proposes that early-universe and late-universe measurements disagree because they sample spacetime from opposite sides of a dimensional phase boundary. The argument develops through eight sections: The Conventional Problem: The 5σ+ discrepancy and the shared assumption that measurements should agree. The Harmonic Unification Framework: Spacetime as harmonically organized rather than smoothly curved, with φ (golden ratio) as a fundamental organizing principle supported by gravitational wave evidence. The Equilibrium Pi Hypothesis: π as the axis of dimensional phase transition—the threshold where systems undergo phase shifts between states. Synthesis: Combining HUF and Equilibrium Pi predicts that measurements across phase boundaries will systematically disagree. The discrepancy ratio (73.0/67.4 ≈ 1.083) falls within ranges predicted by harmonic phase relationships. Predictions and Tests: Intermediate epoch measurements should show intermediate values; directional dependence may reveal transition geometry; redshift-dependent deviations should correlate with phase boundary location. Implications for Cosmological Model-Building: ΛCDM is incomplete rather than wrong; "resolution" attempts may be patching a signal rather than correcting error; the tension may be irreducible. Relation to Other Tensions: S₈, curvature, and age tensions may represent different manifestations of the same phase boundary. Conclusion: The solution is not finding the error but recognizing that the tension itself is the discovery. The water phase analogy clarifies the core insight: measuring water at 99°C versus 101°C produces different results not because thermometers disagree, but because the system has crossed a phase boundary. The Hubble tension may be cosmology's phase boundary signature. DUALITY Protocol Note: This paper was developed through human-AI collaboration, with framework synthesis by the human author and articulation by the AI collaborator.