This paper develops the foundational framework for 5D SpaceTime+ unification, showing that electromagnetic and mechanical inertia arise as complementary projections of the same geometric curvature. By partitioning the invariant rest mass of charged particles into electromagnetic and non-electromagnetic components, the work bridges 4D relativistic dynamics and 5D curvature-based electrodynamics. The formalism provides a self-consistent interpretation of the electromagnetic mass of the electron, connects near-field inertia to field energy, and suggests a unified 5D curvature origin for both charge and inertia. The author gratefully acknowledges the OpenAI ChatGPT platformfor mathematical assistance, LaTeX generation, and research discussion support. Theconceptual development and physical framework are original work by Robert T. Morrow.