Universal Exchange Medium Theory (UEMT): A Classical Framework for Emergent Inertia and Null Propagation from Continuous Dynamical Exchange with a Universal Medium

This work presents Universal Exchange Medium Theory (UEMT), a classical world-line framework based on the hypothesis that every physical system continuously exchanges a dynamical quantity with a universal medium. Within this framework, rest mass, inertial response, and propagation behavior are treated as possible emergent consequences of the instantaneous exchange state rather than as independent fundamental properties. A reparametrization-invariant einbein action is constructed together with a minimal open-system closure, leading to generalized mass-shell constraints, global Lyapunov stability, and a covariant momentum-balance law. The framework predicts that a photon-like state corresponds to a limiting exchange-equilibrium configuration characterized by simultaneous vanishing effective rest mass and net exchange imbalance, producing null propagation. The theory explicitly states its assumptions, known limitations, and falsifiability criteria and remains a classical world-line model pending future continuum and quantum extensions.