This paper introduces Infratier Closure Physics as a closure-theoretic reconstruction of gauge structure, dimensional emergence, and the dark-sector problem within the Unified Coherence Closure Framework (UCCF). Rather than treating U(1), SU(2), and SU(3) as separate gauge factors awaiting reunion at extreme energy, this framework interprets them as successive morphologies of conserved dimensional residue. In this reading, U(1) expresses phase conservation, SU(2) expresses chiral and torsional activation, and SU(3) expresses confinement closure. The paper proposes that the true 3.0D closure object is not merely SO(3), but PSOC₄(3): Phase– Spin/Spatial Orientation Closure, a quaternionically mediated structure in which U(1)-type phase residue and SO(3)-type spatial orientation are unified through the spinorial cover Spin(3) ≅ SU(2). This establishes the first closure block, PSOC₄(3) = 1 + 3, which extends through the infratier sequence PSOC₄(3) + SU(2) + SU(3) = (1 + 3) + 3 + 8 = 15. The framework further distinguishes unprojected coherence, curvature projection, and projected reduction. Within this structure, dark matter and dark energy are reinterpreted as curvature-projection phenomena located at the Continuum Ontology level near 3.14D, rather than as undiscovered Atomic Ontology objects. The result is a generative Big Emergence narrative rather than a Big Bang separation narrative. Coherence projects as curvature, curvature reduces through closure, gauge structure appears as conserved dimensional residue, and matter stabilizes through atomic closure.