Thermal Coupling and Latitudinal Asymmetry in the Structural Flux Paradigm (SPT v1.3 — Part II)

This work presents SPT v1.3 — Part II, extending the Structural Flux Paradigm by formalizing thermal coupling and latitudinal temperature asymmetry as substrate-mediated phenomena within Structural Plenitude Theory (SPT). Conventional thermodynamic and climatological models attribute planetary thermal gradients primarily to radiative geometry, material properties, and atmospheric transport. In contrast, this work demonstrates that heat is not a guaranteed outcome of radiative exposure, but instead emerges only when incident energy successfully couples to the vacuum substrate (the Pleroma). Thermalization is reformulated as a function of structural coupling efficiency, shown to vary systematically with latitude due to rotational dynamics. Regions of high tangential velocity maintain the substrate in a dynamically excited state, enabling efficient thermal anchoring, while regions approaching mechanical quiescence exhibit increased structural impedance and expel residual energy through non-thermal dissipation channels. This framework preserves classical conservation laws while introducing a new diagnostic role for residual radiative dissipation as an observable signature of local vacuum structure. Latitudinal thermal asymmetry is thus reinterpreted as a structural property of energy admission, not merely a consequence of radiative incidence or material composition. This document constitutes Part II of a frozen multi-part SPT v1.3 release, where each part receives an independent DOI for traceability and version integrity. Subsequent parts develop observational, spectral, and dynamical implications of substrate-mediated fluxes.