The present work establishes the foundational field-theoretic structure of the torsion--expansion framework and demonstrates its consistency with known weak-field, Solar-System, and cosmological limits. However, the analysis is intentionally restricted to the construction of the covariant action, the identification of sourced field equations, the derivation of the Newtonian limit, and the formulation of the perturbative framework. A complete mode-by-mode perturbation analysis, explicit strong-field solutions of the coupled system, and detailed confrontation with observational datasets lie beyond the scope of this paper and are reserved for dedicated companion studies. Accordingly, the strong-field sector is parameterised phenomenologically, and the perturbation results are presented at the level of stability structure and programme definition rather than full spectral classification.