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We develop a structural framework for the regularity problem of the three-dimensional incompressible Navier–Stokes equations based on the Fourier-space geometry of triadic interactions. The central idea is that the nonlinear energy transfer can be decomposed into dyadic shell contributions and further classified into Low–Low, Low–High, and High–High channels. Within this decomposition, the potentially dangerous same-scale amplification mechanism is localized to the High–High channel, while the Low–Low and Low–High contributions are shown to be controllable through weighted paraproduct estimates. To quantify the High–High mechanism, we introduce a triadic-family decomposition together with shell-level observables including High–High family transfer, family coherence, coherent time sets, residence times, and shell defect quantities. Using these observables, we formulate a shellwise High–High absorption condition stating that the High–High transfer is dominated by the corresponding viscous scale up to a Sobolev-summable remainder. Under this condition, we prove a conditional regularity theorem: for strong solutions in Hs (3) with s > 5/2, the Sobolev norm remains bounded on any finite time interval, and hence finite-time blow-up does not occur. We then investigate why such an absorption condition is structurally natural. On the Fourier side, we analyze triadic geometry, helical sign structure, and phase dynamics, and show that persistent High–High amplification requires strong coherence and sufficiently long residence in coherent time sets. This provides an integral mechanism suppressing cumulative High–High transfer. On the PDE side, we introduce a relaxation formulation of the Navier–Stokes equations with an independent stress variable and establish a triple-dissipation structure consisting of viscous dissipation, defect-stress relaxation, and stress diffusion. A relative-entropy argument is then used to show that this enhanced dissipative structure is stably transferred to the Navier–Stokes limit. The result is not an unconditional resolution of the global regularity problem. Rather, it provides a precise reduction: the continuation problem for strong solutions is reduced to a shellwise High–High absorption condition with explicit geometric, temporal, and dissipative interpretations. In this sense, the regularity problem is reformulated in terms of the internal structure of the energy cascade, rather than solely by global norm criteria.