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*C^2*R. A PPN analysis yields |gamma-1| ~ 1.28*xi^2, 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).