A Finite Vacuum-Polarisation and Running-Response Doorway for an Electron-Like Packet in the ECSM Framework

This paper presents the V28 continuation of the ECSM electron-like packet programme. It tests whether the inherited packet chain can support a finite vacuum-polarisation and running-response doorway. The construction introduces a response-regulated polarisation function that tracks ordinary low-to-moderate momentum leading-log running behaviour while saturating at high probe momentum through finite ECSM response. The notebook reports exact inherited alpha-beta-sigma algebra, low-q reference tracking, high-q response saturation, finite coherence deformation, and a no-fit/no-overclaim audit. The final result is labelled: PASS_STRONG_VACUUM_POLARISATION_DOORWAY This is not claimed as full QED, a precision running-alpha calculation, a renormalisation-group completion, or a scattering-amplitude derivation. It is a finite doorway-level result showing that the ECSM electron-like packet can reproduce low-energy running-response behaviour while avoiding uncontrolled high-energy logarithmic divergence through finite medium response.