Shell-Localized Gravitational Response and Horizon Formation in a Finite-Response Medium

We present a finite-response framework for gravitational dynamics in which compact objects are described as saturated cores surrounded by gradient-dominated shells. In this model, gravitational response is determined by spatial gradients of the medium rather than mass concentration alone. Horizon formation occurs where propagation speed is reduced by strong gradients, leading to a shell-localized trapping surface. The framework provides a unified interpretation of compact object structure and is consistent with observational features such as ring-like emission in black hole imaging and boundary-dominated dynamics in neutron stars and accretion systems. A key prediction is that horizon location depends on internal structure rather than enclosed mass alone, providing a testable distinction from standard geometric descriptions of gravity.