CDN-242 · The Residual Reduction — GCFT Dark-Sector Book Arc (Art Edition)

CDN-242 executes the three avenues pre-registered by CDN-241 on the validated 25-cluster sample, reusing the CDN-240 locked physics bit-identically. A refined seven-term κ-proxy with an intermediate-λ curvature term (b₆ = +21. 98, centred on ln λ₀ = ln 17. 5) lowers the mean proxy error from 0. 041 to 0. 029, cuts the dominant Class-S (sterile reservoir) residual mass by 43. 3%, reduces the total residual budget by 16. 9% (30. 93 → 25. 72 M̄), and raises mean dark-sector closure from 96. 6% to 97. 2%. The same refinement does NOT reduce the Class-P collisionless-offset residual — it rises (5. 16 → 7. 83) — confirming that residual as physical rather than a proxy artefact. A formation-history proxy φ cleanly separates the two Class-F clusters (φ̄ = 3. 58, both super-ceiling with κᵣeq > 1) from the equal-κᵣeq Class-S interloper CL 0024+17 (φ = 0), establishing Class F as a genuine super-ceiling relaxed population. The surviving Class-P residual bounds the non-GCFT (particle-like) dark-sector fraction to ≤ 0. 29% population-averaged (≤ 10. 1% in the most extreme single offset merger) ; the GCFT coherence floor is ≥ 99. 71%. The dark-sector residual therefore splits into a refinable part (S, proxy-resolution noise) and an irreducible part (P physical offset; F super-ceiling). Art Edition with star-field design system (seed 242). Lock: CDN242RESIDUALREDUCTIONLOCKED.