T109 replaces the ledger treatment of the factor \(G=9\) from T105 with a structural decomposition derived from the leakage and boundary architecture developed in T106-T108. Rather than entering as an assumed generation/boundary symmetry multiplier, \(G\) is expressed as \[G = 3_{\mathrm{triad}} \times 3_{\mathrm{boundary}} = 9,\] where \(3_{\mathrm{triad}}\) arises from the effective leakage-fibre reduction and \(3_{\mathrm{boundary}}\) arises from the rank-three boundary interaction space \[\mathcal{B}_Y=\operatorname{span}\left\{\Pi_Y,\,K,\,K^\dagger e^{tB}K\right\}.\] Using this decomposition, the mirror-to-baryonic geometric ratio becomes \[\rho_{MB}=\frac{|B_5|\,G}{|Q_5|\,D\,F}\cdot\frac{160}{161}.\] Substituting the established geometric quantities \[|B_5|=3840,\qquadG=9,\qquad|Q_5|=32,\qquadD=8,\qquadF=25,\] yields \[\rho_{MB}=\frac{3840\cdot9}{32\cdot8\cdot25}\cdot\frac{160}{161}=\frac{864}{161}=5.366459627\ldots\] recovering the same numerical value obtained previously in T105. The significance of T109 is structural rather than numerical. The factor \(G=9\) is no longer a bare ledger assumption but is resolved into independently motivated geometric components derived from the leakage triad and the boundary interaction space. This shifts a major portion of the ratio from phenomenological bookkeeping into the internal architecture of the Q5 framework. After T109, the principal remaining unresolved denominator factor is \[D=8,\] whose derivation is deferred to T110. Candidate origins include the tesseract boundary structure, dual-tesseract orientation splitting, visible/complement sector normalization, or the \(Q_5=Q_4\times\{0,1\}\) decomposition yielding eight cubic boundary cells. Epistemically, the arithmetic and the resulting value \[\rho_{MB}=\frac{864}{161}\approx5.3665\] are solid given the stated inputs. The structural decomposition of \(G\) remains conditional on the independence result established through T107-T108, while the geometric grounding of \(D=8\) remains the next open derivation target in the ratio program.