FrostGrid Electromagnetism: A Local Reversible Weak-Field Derivation of Maxwell-Like Source, Wave, Energy, Momentum, and Force Structure

FrostGrid Electromagnetism develops a Maxwell-like weak-field sector from the previously published FrostGrid Fundamental Mechanics v1.0. The goal of this work is to test whether the mechanical rules of the FrostGrid can support the basic structures needed for electromagnetic-like behavior without introducing electromagnetism as a separate fundamental substance. Within an explicitly declared weak-response branch, the paper develops two transverse propagating modes, a Gauss/source sector with exact source continuity, positive field-energy flow, field momentum and stress, signed-source attraction and repulsion, a moving-source circulation response, a Lorentz-force-like material load, exact non-radiative uniform source motion below the causal speed, and a transverse radiation channel. The two weak response variables are identified by their derived roles as an electric-like source/work partner and a magnetic-like circulation partner. This work establishes the field-side groundwork needed for later FrostGrid matter models. Rather than beginning future charged-particle work by guessing electromagnetic field equations, the paper provides a self-consistent Maxwell-like weak sector against which candidate FrostStructures can be constructed and tested. The remaining matter-side problem is to determine which microscopic FrostGrid structures can realize stable signed sources, how complete FrostQuanta combine into charged FrostStructures, and how the resulting FrostGrid source strength maps to observed elementary charge. The work does not claim a complete microscopic derivation of observed electromagnetism. The exact FrostLink constitutive law selecting the weak branch, the microscopic structure of charged particles, the conversion between a FrostGrid source quantum and observed elementary charge, absolute radiation and matter-response strengths, physical spatial winding, and several complete-FQ response quantities remain open. These unresolved quantities are kept explicit rather than fitted to known electromagnetic measurements. The accompanying publication package includes the paper, source files, glossary, provenance map, checksums, and a research/reproducibility archive covering the test sequence that led to the final weak-sector closure. The work is presented as a speculative, reproducible theoretical framework rather than experimental confirmation of the FrostGrid as a model of nature.