The Three-Channel Taxonomy of Light–Gravity Coupling: A Bipartite Reading of Einstein–Maxwell

Preprint of the manuscript submitted to Foundations of Physics on 10 May 2026 (passed technical checks 2026-05-13). Proposes a clean three-channel taxonomy of light–gravity coupling in the linearised limit of general relativity coupled to electromagnetism: gravitoelectric (mass-induced redshift, lensing, time dilation), gravitomagnetic (rotation-induced Lense–Thirring effect, gravitational Faraday rotation), and Λ-cosmological (cosmological redshift, integrated Sachs–Wolfe effect, de Sitter horizon thermodynamics). Headline falsifiable prediction: the Skrotskii–Mashhoon polarisation rotation Δχ ≈ 35 μrad at b = 100 R_S, a* = 0.7, detectable at ≳ 16σ/campaign by ngEHT around 2030. Companion computational audit with 56 simulators + 836 unit tests at https://github.com/LeKCei/bipartite-universe-corpus.