This paper provides the unified ontological foundation for the Mutual Information Density Hypothesis (MIDH) series. It consolidates and clarifies the structural commitments underlying five prior papers on entity persistence, meta-evolution of physical laws, cosmic acceleration, quantum entanglement, and chronology-first ontology. The central purpose is to close an interpretive ambiguity: MIDH is a structure-first framework, not a quantum-first one. The paper explicitly states that classicality is ontologically real – not an epistemic illusion, not a perspectival slice of a branching multiverse, not a "for all practical purposes" approximation. Quantum and classical behaviors are different structural phases of the same underlying correlation dynamics, distinguished by mutual information density and meta-coupling strength. The paper introduces the core primitives (correlation, chronology, metas), formalizes the viability law (R = D_int / D_env ≥ κ), and positions MIDH relative to existing interpretations of quantum mechanics. A dedicated section explicitly rules out common misreadings: not Everettian, not QBist, not Bohmian, not retrocausal, not consciousness-dependent, not decoherence-only. Intended as a reference document for readers engaging with any paper in the MIDH series, it establishes the shared conceptual backbone and identifies open research questions including Born rule derivation, meta-transition dynamics, and mathematical formalization.