Intrinsic Response Sector as Dark Gravity: A GR-Compatible Candidate Identity for the Cold Dark Matter Role (SPARC-175)

This preprint develops a falsification‑first, GR‑compatible alternative to particulate cold dark matter by defining an intrinsic response sector—an effective stress–energy component Tμνresp inferred directly from galaxy kinematics. Working within standard general relativity, the framework identifies the minimal response required so that observed weak‑field potentials satisfy the Einstein equation with baryons alone. A single‑parameter boundary‑layer closure tied to a baryonic transition radius reproduces the characteristic 1/R outer‑acceleration tail and yields strong improvements over baryons‑only fits for 92% of 175 SPARC galaxies, with 97% passing BIC overfitting tests. The baryonic Tully–Fisher relation emerges as a consequence of the closure, and robust agreement between model‑dependent and model‑independent amplitude estimators supports the interpretation of a genuine outer‑region response. The intrinsic response sector is presented not only as a phenomenological reconstruction but as a candidate identity for the cold dark matter role, contingent on forthcoming lensing and cosmological discriminants. The paper provides explicit falsification criteria, a domain‑partitioned validation ladder, and a branching structure (metric‑equivalent vs. slip‑allowed) to guide cosmological completion.