The Third Verdict in a Curved Background: Is the Modular Orientation of a Comoving Detector Irreducible to Gibbons–Hawking Thermality? (Phase VI)

We extend the programme's modular-orientation decision to a fixed, semiclassical 1+1 FRW background (no dynamical gravity, no backreaction, no quantum gravity). For a comoving detector we build the spatial interval modular Hamiltonian on a constant-time slice from the genuine adiabatic-vacuum mode functions, using the bosonic correlation-matrix construction; its exact canonical symplectic form makes the continuum limit numerically stable, unlike the timelike worldline construction, which does not converge under refinement. A symmetric, preregistered discriminator compares the FRW interval to the de Sitter Gibbons–Hawking thermal reference family, normalised by a genuine temperature-mismatch unit so that the universal boundary divergence cancels. The apparatus is validated: de Sitter states sit on the floor and recover TGH = H/2π to 0. 02%, while a non-thermal foil registers three orders of magnitude above the floor. The blind sweep returns HORN-A graded (ii) in 10/12 cells and COLLAPSE in the near-de Sitter / short-interval corner: the modular orientation is geometric and present, yet a functional of the classical expansion history (an evolving Gibbons–Hawking temperature), reducible to known time-dependent thermal QFT — not a new arrow. A dedicated grade- (i) einselection test is shown to be a particle-production readout and is recorded as a false positive averted by scrutiny; we argue that genuine grade (i) is structurally unreachable in fixed-background free-field theory. Scope. Fixed classical background; no dynamical gravity, backreaction, or quantum gravity. Archived as Phase VI (exploratory, curved background) ; not merged into the Minkowski Phase IV/V corpus. Preregistration. Instrument, controls, production configuration, and verdict rule were hash-frozen before the blind sweep (2026-06-30; SHA-256 of the immutable block: 8b13a5ef2f7003a3bf11442847a520b9a980673ea260c7cb29627515bedfd29a), with an append-only deviation log (D1–D3).