This work presents version 1.8 of Structural Differentiation Cosmology (SDC), a cosmological framework in which the observed cosmic acceleration is interpreted as an observational projection of irreversible internal structural differentiation within an externally closed universe. Unlike the standard ΛCDM model, SDC does not introduce dark energy or additional energy components. Instead, the framework predicts a characteristic separation between background expansion geometry and the growth of cosmic structure. While the background expansion history remains nearly degenerate with ΛCDM, the growth observable fσ8(z) exhibits a systematic percent-level suppression. We introduce a geometry–growth diagnostic plane that provides a direct visual separation between projection-based acceleration and physical acceleration. The framework therefore provides a minimal and explicitly falsifiable observational pathway for testing SDC with upcoming large-scale structure surveys.