NT17-A: Stability Map in (e,n) Space for the Oscillatory Parametrisation

We compute a two-phase stability map of the COCM oscillatory parametrisation in the space of orbital eccentricity e and dataset size n. Using mock catalogs with Gaussian noise (A_0=0.091, φ=−0.956), we measure the fraction f_S of free optimisations that recover ω_z = 11.1991±1.0. The regime boundary is found at n_crit = 12.8 ± 1.5, nearly independent of e. DR2+CC (n=14) lies marginally above n_crit under Gaussian noise — the non-recovery observed in real DR2 data is not primarily a statistical-resolution effect but reflects displacement by a compact high-tension substructure (ELG2, −13.4σ). This result formally revises the qualitative interpretation of NT16 §2.