Dynamic Harmony Structural Stress-Test Series. This paper examines the structural boundary between internal system organization and external environmental interaction. The analysis evaluates whether architectural closure provides a reliable criterion for distinguishing systems that possess internally defined organization from those whose apparent structure is imposed by external conditions. Using the Phase Non-Substitutability Test introduced in Paper 0 and the architectural boundary criterion developed in Paper 0B, the paper analyzes the conditions under which a system establishes a genuine internal architecture capable of sustaining coordinated organization across perturbations. The analysis clarifies the structural requirements for architectural closure and examines how internal–external boundaries constrain the emergence of organized systems. This paper forms part of the Dynamic Harmony Structural Stress-Test Series, a research program that evaluates major theories of emergence across physics, biology, and complex systems through adversarial structural analysis.