The Participatory Decoder: Architecture for the Participatory Horizon Programme - Part III

Persistent large-angle anomalies in observational cosmology, including suppressed CMB correlations beyond 60^, excess source-count dipoles, and quadrupole--octopole alignment with the kinematic dipole and planarity (`Axis of Evil'), share an observer-dependent character that primordial mechanisms should not produce. The participatory-horizon programme proposes that these anomalies arise because the observer's causal diamond acts as an information aperture whose geometry constrains which correlations are realised as classical records, while the standard CDM cosmology is retained in full. The companion PMH framework~OGradyPMH2026 derives an angular recording filter h² () and a radial kernel kL (x) from the conformal geometry of the causal diamond, and the companion Lindblad paper~OGradyLindblad2026 constructs the local irreversible recording dynamics with canonical collapse operator Rₐ = ₐ/W. Both papers identified a variance channel, m-dependent modulation from anisotropic position-space recording, but deferred its future quantitative development. The central question addressed is whether position-space recording on the causal diamond can produce the observed quadrupole--octopole alignment and low- planarity, and if so, what the framework predicts for the pattern of m-dependent variance modulation in the low- CMB sky. The framework yields exact, parameter-free variance coefficients, ₂₀ = 2/7, ₂₁ = 1/7, ₂₂ = -2/7, locked to the kinematic dipole direction. The decoder concentrates Fisher information on zonal (m = 0) modes, suppressing their posterior variance while leaving sectoral (|m| =) posterior variance relatively enhanced, thereby producing the geometry of the planar morphology observed in the CMB quadrupole. A parity selection rule, following from the quadratic structure of the Lindblad hazard, forces the canonical decoder to be antipodally symmetric: it cannot produce a diagonal even/odd C_ parity asymmetry. A common-axis theorem shows that independent bias of the quadrupole and octopole axes toward v enhances their mutual alignment without requiring direct = 2 3 covariance coupling. The amplitude falls short of the observational target by 15--17 orders of magnitude, and this gap is shown to be structuralwithin the canonical recording architecture. The geometric predictions, i. e. , the ₌ variance pattern, the parity selection rule, and the common-axis theorem, hold regardless of the amplitude and constitute parameter-free predictions of the framework.