The increasing penetration of renewable and inverter-based resources is reducing operating margins and increasing the complexity of power-system security assessment. This paper introduces a geometry-conditioned entropy framework for assessing operational fragility under the DC load-flow approximation. The feasible operating region is represented as a convex polytope in reduced injection space, enabling the definition of direction-independent distances to feasibility boundaries and associated local robustness margins. The framework combines a geometric distance-to-infeasibility measure with three complementary entropy-based indicators: flow entropy, redispatch-sensitivity entropy derived from PTDF coefficients, and a Distance-Weighted Redispatch Diversity Index (DWRDI) that incorporates proximity to feasibility boundaries. Application to the IEEE-39 bus system shows that shrinking feasibility margins can be detected before any constraint activation occurs. As feasibility margins contract, system behaviour becomes progressively governed by a decreasing number of feasibility facets, eventually a single limiting constraint. The proposed framework provides a geometry-based early-warning perspective on declining operational flexibility and the evolution of constraint dependence while all operating limits remain satisfied.