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, VB), where VB (C) = mu⁴ (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 OmegaDE ~ 0. 70 and Cₜoday ~ 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).