Ringdown Gravity: A Self-Contained Effective Gravitational Theory of the Dark Sector

Ringdown Gravity (RG) is a self-contained effective gravitational theory in which phenomenology conventionally attributed to dark matter and dark energy is encoded as a state-dependent gravitational response rather than a new dark particle.The fixed-background foundation is retained from the previous versions: in the axial l=2 Schwarzschild sector, a canonically normalized horizon distinguishability functional coincides, after the affine horizon map, with coherent modular relative entropy, D_H = S_rel. For an exact single pole the information decays at twice the field damping rate, and the same affine future kernel extends to stationary non-extremal Killing horizons once the appropriate generator and canonical flux are supplied.Version 3 corrects the nonlinear cosmic-void interpretation. A dynamical-void audit shows that the version-2 clock activation chi = Y/(Q^2+Y) cannot by itself transmit the reciprocal endpoint response to an expanding cosmic void. Version 3 therefore separates accumulated signed History depth from quasistatic clock activation at action level. The local constitutive primitive is a history-weighted composition of the existing neutral, cluster-endpoint and void-endpoint primitives, supplemented by a minimal high-acceleration tangent term that preserves the static state-support normalization without adding a new observational fit parameter.The resulting one-metric effective action has explicit metric, History-current and clock Euler equations and retains the on-shell Ward/Bianchi identity. The new closure remains elliptic on the tested local half-turn chart, reproduces the previous Solar-System and compact-star static-support benchmarks, and leaves the local action at O(delta^4) around FLRW, so the existing background and linear-CMB regression is unchanged.The reciprocal relation is retained as a local maximally developed void endpoint: R_void^local(s=-1) = 1/R_c < 1. For the benchmark R_c = 1.90 this endpoint is 0.526315. Version 3 explicitly retracts the stronger statement that every expanding cosmic-void stack must have projected lensing amplitude equal to this number. Under an explicitly stated adiabatic History-tracking approximation, applying the version-3 closure to published least-squares Full, Small and Large UNIONS+BOSS void profiles gives conditional projected DeltaSigma_RG/DeltaSigma_GR amplitudes of approximately 0.903, 0.939 and 0.965. These values are consistency forecasts, not likelihood constraints.The accompanying reproducibility package contains the manuscript source/PDF, the inherited Schwarzschild, CAMB, variational, spherical/PPN and state-support scripts, and a new version-3 script reproducing the history weights, ellipticity scan, static-support regression and conditional published-profile void-lensing calculation. Correspondence: [email protected]