Variational Backbone and Regime Closures XVIII (Dark Sector): The Dark Residual Source Mass and Dark Residual Sources from One Comparison-Representative Hessian

This paper treats the dark residual source in VBRC as a metric footprint of licensed unread content, not as an independently inserted dark-matter Lagrangian or bare dark-energy term. It is not a cosmological data-fitting rail and does not rederive gravity. The Part X metric gate is inherited. The task of Part XVIII is the source-entry side of the theory: to show how licensed unread summaries can become mass-like, gravitational-source-like, dark-matter-like, dark-energy-like, and Lambda-like readouts without inserting primitive dark fluids. Starting from the Part I comparison-density chain, unread content enters the retained metric law only through a licensed full summary and through a shifted displacement relative to a chosen background summary. For a fixed metric, the open variable is relaxed, producing a background-normalized residual energy and its metric stress. In this shifted representative, the zero displacement carries no residual dark energy by definition, so the dark source is a readout of the shifted summary residual rather than a raw vacuum term. The central construction separates two Schur directions of one comparison-representative Hessian. The mass readout is the open-Schur gap, while the dark gravitational source is generated by the opposite dual-Schur residual. Thus mass and dark source are not independent additions; they are opposite reductions of the same retained-summary Hessian. A cosmological spectral gate then splits the shifted summary into infrared and inhomogeneous components. The inhomogeneous component is dark-matter-like only under positive-density, pressure-suppressed, clustering gates. The infrared component is dark-energy-like only under metric-proportional and slow-variation gates. The Lambda-like term is the metric-proportional infrared projection of the same reduced stress, not a raw zero-point vacuum sum. The main structural output is a non-independence theorem: admissible dark-matter-like and dark-energy-like readouts must lift to one dual-Schur residual source and one hidden-summary closure load. At the same quadratic level, the retained Schur gap and the dual-Schur source satisfy determinant and index constraints. In a scalar active-channel representative, their relative conversion has a normalized Lorentzian shape, while the source amplitude and critical scale retain coupling dependence. These are structural constraints on dark-sector readouts, not numerical fits of cosmological density parameters or an equation of state.