CDN-237: The Dual Register — Residual-Matter Taxonomy and Observational κ-Proxy Specification

CDN-237 resolves the CDN-236 fork by taking both branches simultaneously. Track A classifies every unit of residual R/M̄ into a four-class taxonomy {P, F, S, E}: particle-like dark matter, field memory, sterile reservoir, or effective unclosed sector. Track B specifies a sigmoid κ-proxy that estimates the coherence-order parameter from weak-lensing mass, gas fraction, topology label, and ICM morphology indicators. A five-rule contract (C1–C5) binds CDN-238 to satisfy whichever branch proves stronger on real data. At λ=20, zero residual matter requires κ=0.851. Neither branch closes the dark sector; residual matter stays explicit until κ is independently earned.