Gravitational Sector Constraints on Coherential Dark Energy

We investigate the gravitational sector of the P2 dark energy action — a canonical scalar field C with exponential potential — motivated by universal quantum decoherence (Papers I, II). We test whether C can couple directly to curvature through a non-minimal term xi*C²*R. A PPN analysis yields |gamma-1| ~ 1. 28*xi², and the Cassini bound imposes |xi| < 4. 2e-3. Within canonical P2 no known screening mechanism relaxes this constraint. A strong-field test on a fixed neutron-star background confirms regularity within the Cassini window up to compactness M/R = 0. 369. We adopt minimal coupling (xi=0) as the default stance, falsifiable by future gravitational tests. Version 2. 0 aligns notation and conventions with the full RCD disformal series (Papers IV-XVII).