CDN-243 · The Holdout Pre-Registration — GCFT Dark-Sector Book Arc (Art Edition)

CDN-243 pre-registers the out-of-sample test instructed by CDN-242. The seven-term κ-proxy locked by CDN-240/241/242 (b₀..b₆ with b₆ = +21.98 at ln λ₀ = ln 17.5), and the saturation block (ξ_target = 5.836, ξ_gal = 5.851, S_eff = 0.0623, ε_sat = 0.068), are propagated bit-identically to an independent CCCP / Weighing-the-Giants cluster sample. The population-averaged Class-P bound on the non-GCFT dark-sector fraction (≤ 0.29% mass-weighted, ≤ 10.1% in the most extreme single offset merger) is converted into a per-cluster prediction template: for every holdout cluster the locked proxy assigns a class (S / F / P / E), a predicted κ_req, and an upper bound on its residual R/M̄ with class-dependent bandwidths (S ±0.10, F ±0.20, P ±0.30, E ±0.05). The decision rule is fixed in advance: PASS if ≥ 80% of clusters fall within their predicted bands and the population-averaged non-GCFT bound holds within 0.10 percentage points; CONFLICT if either fails by ≤ 2×; FAIL (CDN-242 P-bound REFUTED, theory-debt D9 opened) if the bound is broken by ≥ 2×. Three falsification pathways are enumerated: mass-class failure (proxy incompleteness), P-bound violation (genuine non-coherence dark matter under-counted by CDN-242 selection), and Class-F sparsity (Class F may be a CDN-240 selection artefact). Independent-sample selection rule is locked: disjoint CCCP-extension (Hoekstra+ 2015) + WtG (von der Linden+ 2014, Applegate+ 2014); M₂₀₀ ∈ [3, 25] × 10¹⁴ M☉; weak-lensing fractional uncertainty ≤ 30%; X-ray T available for relaxedness flag; minimum N ≥ 15 or CDN-244 reports inability-to-test. An illustrative 16-cluster candidate list is given but not frozen — CDN-244 must publish a final frozen sample with all exclusions justified before the locked proxy touches any of them. No new computation. No refit of b₀..b₆. No promotion of the ≥ 99.71% GCFT closure floor to a cosmological constraint. Art Edition with star-field design system (seed 243), built on art_base.py codifying the GCFT Art Edition Layout Reference. Lock: CDN243_HOLDOUT_PREREG_LOCKED.