Boundary-Mediated Transport Doubling and Emergence of the 1/160 Defect Scale

This paper establishes the mechanism by which the observable phase defect \ μ = 1/160 \ emerges from a one-way mediated crossing asymmetry at scale ε = 1/320. Under outer-to-outer transport with no direct first-order coupling (all transport passing through a middle mediating layer) each crossing leg contributes an antisymmetric generator A at scale ε. The round-trip crossing-return operator produces an antisymmetric residual at leading order: \ μ = 2ε + O(ε²) \ With the minimal mediated suppression scale ε = 1/320 (the r=1 slot, the least suppressed primitive mediated channel), the observable defect at leading order is μ = 1/160, matching the theorem-chain coefficient. The readout projection \ ½Tr(·R) \ annihilates symmetric and scalar contributions, retaining only the antisymmetric A-component, so only the doubled antisymmetric part of the round-trip operator contributes to the observable. The doubling mechanism itself is solid. The identification \ ε = 1/320 \, specifically as boundary asymmetry, is conditional on an explicit derivation that Q5 boundary geometry forces r=1 as the operative slot. Resolving that open item would upgrade TA5 from a coherent mechanism to an inevitable structural consequence of Q5 geometry. The minimal mechanism for the defect requires four elements: a small one-way asymmetry ε, a preserved antisymmetric carrier A across both legs, two-leg transport, and readout projection.