The Einsteinian Stack Closure Theorem establishes the final, complete closure of the Einsteinian Stack by proving that the primitive set defined across Papers 1–29 is sufficient, minimal, non-redundant, and non-extendable. The theorem demonstrates that all admissible dynamics involving distinguishability, operators, curvature, observers, semantics, gates, drift, and termination reduce uniquely to this primitive set. Any higher formulation, including canonical preimage representations, is shown to be an isomorphic reparameterization rather than an extension. This paper formally seals the Stack as a complete geometric system.