Robustness of the EFC Growth-Sector Signal: Leave-One-Out, Sound-Horizon, and Amplitude-Prior Controls for the Hubble-Friction Channel

Energy-Flow Cosmology (EFC) modifies the Friedmann equation through an entropy-gradient coupling term governed by a single late-time parameter α. A joint Bayesian fit to 14 BAO distance measurements and 7 fσ₈ growth-rate data points yields α = −1.00 ± 0.46 (2.20σ) under controlled nuisance priors (N2a mode: r_d = 147.09 Mpc, σ₈ ∼ N(0.81, 0.02)). This note subjects the signal to three independent robustness controls: (N1) sound-horizon independence, (N2) amplitude-prior sensitivity, and (T7) leave-one-out removal of each fσ₈ data point. Result: all 7 LOO runs pass (|α|/σ ≥ 1.84 and ΔAIC ≤ 0), the signal range is narrow (α ∈ [−1.11, −0.88]), and parameter correlations are stable. The ∼2σ hint for late-time growth suppression is not an artefact of calibration choices, prior widths, or any single measurement. It is a distributed, robust, sub-detection-threshold signal awaiting confirmation or falsification by forthcoming surveys.