V90R: Σ–Π Coupled Update Dynamics with SU(N) Structure Constants — A Dynamic Window Explanation for SU(3)=8

We connect V90's "QCD as window specification" to the YAGC V84 Σ-update engine by embedding SU(N) structure constants into a minimal Σ–Π coupled dynamics. We prove that the naïve Σ-only non-abelian friction term (fabcΣbΣcf^{abc}\Sigma_b\Sigma_c fabcΣbΣc) vanishes identically due to antisymmetric × symmetric contraction, explaining prior non-abelian necessity test failures as a specification boundary rather than implementation error. Introducing a two-variable Σ–Π model where fabcΣbΠc≠0f^{abc}\Sigma_b\Pi_c \neq 0 fabcΣbΠc=0, we scan SU(2), SU(3), SU(4), and SU(5) under identical normalization. Dynamic diagnostics—effective coupling matrix spectral radius ρ(M) and perturbation growth rate γ—reveal a "Goldilocks" regime: SU(3)=8 achieves the lowest spectral radius (0.84) and growth rate (0.0007), making it dynamically most stable under driving variation. In contrast, SU(2) is too uniform (fragile) and higher SU(N) develops unstable modes. We conclude that SU(3)=8 can be interpreted as an emergent stability optimum of coupled Σ-update channels under non-abelian structure, bridging the V90 window-specification view and the V84 update engine. This supports the YAGC principle of Information-Structural Necessity (ISN): fundamental window structures may be constrained by sustainable update dynamics.