Relativistic Shedding: A Kinematic Threshold for Emission into Weakly Coupled Eigenmodes (Foundational formulation, Version 6)

Relativistic Shedding (RS) is defined here as a threshold turn‑on of energy transfer from a relativistic, constant‑velocity source into a propagating eigenmode of the coupled field–environment system. The defining condition is purely kinematic: emission becomes allowed when the source synchronism line intersects a propagating mode branch (equivalently, a phase‑velocity criterion in bulk media). This paper formalizes RS independently of any single laboratory architecture and provides minimal, model‑independent falsifiers (threshold bracketing/ABAB reversibility, resonance selectivity, detuning controls). It supersedes earlier exploratory RS drafts by consolidating a stable definition and a compact formalism intended to remain valid across future refinements in implementations, parameterizations, and cosmological applications.