This paper formalizes the architectural separation between pure computation and governed mutation in protocol-governed systems. Building on the execution model defined in Paper 6 [Bachi, 2026f], it specifies the semantic boundary between Capability Transforms (CT_) and Capability Side Effects (CS_). We define purity invariants for transforms, mutation declaration requirements for side effects, ordering guarantees, replay constraints, and mutation surface bounding. This separation is shown to be necessary for deterministic replay, trace completeness, and structural enforceability. This paper does not analyze security implications (Paper 8), execution engine architecture (Paper 6), governance mechanics (Paper 4), or lifecycle economics (Paper 9). Its purpose is definitional: to formalize the world-interaction boundary that enables determinism and auditability guarantees.