Linear Stability of Coherential Dark Energy under Scalar Perturbations

We investigate the linear stability of the P2 dark energy action — a canonical scalar field C with exponential potential, motivated by universal quantum decoherence (Paper I). The Lagrangian structure guarantees ghost freedom (K=1>0), luminal propagation (c^2_s=1), and absence of scalar anisotropic stress. On the self-consistent P2 background (beta~22, Omega_DE~0.70), the effective mass satisfies m^2_eff~4H_0^2>0 (no tachyonic instability). A full 8-variable numerical solver confirms stability across super-horizon, horizon, and sub-horizon regimes for both adiabatic and isocurvature initial conditions. All six tests pass. Version 2.0 aligns notation and conventions with the full RCD disformal series (Papers IV-XVII).