GCFT CDN-176: Two-Parameter Convergence Surface — ε(ξ,θ) ≈ 0.83·ξ⁻⁰·⁸⁰·exp(−0.017·ξ⁰·⁴¹·θ), D8 Approximate, σ=16

CDN-176 maps the two-parameter convergence surface ε(ξ,θ) where ξ = Δ·σ² and θ = T/τ_local. 75 simulation pairs at σ=16, Δ∈{0.020,0.060,0.100}, θ∈{5,10,15,20,30}. Exponential decay ε=ε₀·exp(−κ·θ) confirmed (R²≥0.93, best 0.9955). Parameter power laws: κ(ξ)≈0.017·ξ^0.41, ε₀(ξ)≈0.83·ξ−0.80. Combined convergence law: ε(ξ,θ)≈0.83·ξ−0.80·exp(−0.017·ξ^0.41·θ). Self-consistent with CDN-173/174 θ=20 data to <0.3%. D8 awarded approximate; CDN-177 pre-registered for stretched-exponential refinement. Lock code: CDN176_CONVERGENCE_APPROXIMATE_D8_EXP_DECAY_XI_DEPENDENT_KAPPA