FLD with derived flux; moving cases include first-order gas/radiation evolution and lab-frame enthalpy transport. Available analytic references solve qualified linear diffusion-limit problems; their discrepancy norms cover the projected slice, not the full volume. Thin positive-opacity streaming cases have no supplied analytic spatial FLD reference and instead check exact periodic energy integrals.
No analytic spatial FLD reference is supplied; exact periodic energy integrals and physical bounds are checked. Flux remains derived; no separate momentum conservation test applies.
Central cell layer at z=dx/2; ray at y=z=dx/2.
