GCFT CDN-179: Global Fine-Grid Convergence Surface — Corrected Two-Parameter Law ε(ξ,θ), D8 Full Global, σ=16

Fine-grid rerun of the drag-peak convergence surface across all three ξ values (5.12, 15.36, 25.60). 45 new simulations on v-grids with spacing ~0.007, combined with CDN-178/177 data. The floor model ε = ε∞ + εt·exp(−κθ) achieves R² ∈ {0.997, 1.000, 0.996}, confirming a nonzero asymptotic floor identified as acoustic phase-velocity dispersion at k ~ 1/σ. Corrected global law: ε(ξ,θ) ≈ 0.78·ξ^(−0.85)·exp(−0.039·ξ^(−0.37)·θ). Supersedes CDN-176 (κ exponent sign reversed from +0.41 to −0.37; coarse-grid bias confirmed globally). Theory debt D8 paid in full globally. Supplement to CDN-178.