The P2 Action: Construction and Cosmological Validation of Coherential Dark Energy

We construct a dark energy action motivated by the hypothesis that dark energy is the macroscopic residue of universal quantum decoherence. Starting from an ontological framework (Radial Coherential Dynamics, RCD) in which a scalar field C represents a coarse-grained coherence order parameter, we derive the most general homogeneous action in FRW spacetime and subject all candidate kinetic terms K(C) and potentials V(C) to a systematic filter pipeline requiring domain consistency, structural irreversibility, ghost freedom, and falsifiability. Of four candidate pairs, only one survives all filters: P2 = (K=1, V_B), where V_B(C) = mu^4(1 - exp(-beta(1-C))) with beta ~ 22 and alpha ~ 1.3e-4 is the proposed universal decoherence rate. Self-consistent numerical integration in FRW reveals a cosmological attractor with Omega_DE ~ 0.70 and C_today ~ 0.80, emergent irreversibility without time-asymmetric terms in the Lagrangian, and an equation of state w0 ~ -0.987 (thawing quintessence). Version 2.0 aligns notation and conventions with the full RCD disformal series (Papers IV-XVII).