A Schwinger-Scale One-Loop Magnetic-Moment Doorway for an Electron-Like Packet in the ECSM Framework

This paper presents the V25 stage of the ECSM electron-like packet programme. Building on the V21b--V24 chain, the notebook inherits a near-electron rest-energy packet scale, coherent-limit Dirac-like propagation, an emergent gauge-coupling doorway, and the Pauli-limit tree-level magnetic coupling g = 2. V25 tests the next structural layer: whether a one-loop magnetic response kernel can generate the Schwinger-scale anomalous magnetic-moment doorway, a_loop = alpha_fs / (2 pi), without fitting to the measured anomalous magnetic moment. The notebook recovers: a_loop = 0.0011614097328858596 and g_one_loop = 2.0023228194657716, with the one-loop spin-splitting correction matching the same relative factor to numerical precision. The measured anomalous magnetic moment is not used as a fit input. This is not claimed as a full QED derivation. The paper does not claim full renormalisation, Lamb shift, scattering amplitudes, higher-loop corrections, or complete radiative QED. It establishes a Schwinger-scale one-loop magnetic-moment doorway for the ECSM electron-like packet and identifies the next required stage: propagators, vertex structure, loop regularisation, self-energy, Lamb shift, and scattering amplitudes.