A WebGPU port of Geant4-DNA, the CNRS/IN2P3-coordinated Monte Carlo track-structure toolkit for radiobiology. One GPU thread per primary electron, the full particle history in a single fused WGSL compute dispatch, Karamitros 2011 9-reaction Independent Reaction Time chemistry in a Web Worker, and event-level SSB/DSB scoring on a 21x21 B-DNA fiber grid at 10 keV. Validation against a Geant4 11.4.1 / G4EMLOW 8.8 dnaphysics ntuple is committed as falsifiable JSON artifacts under experiments/results/ — see README § Numbers for the single source of truth.