The FGOC (Focal-Geometry and Curvature classifier) is a lightweight, deterministic module for early identification of non-Keplerian and potentially interstellar trajectories in LSST DIASource short arcs. The module computes four geometric–curvature diagnostics directly from 2–5 point astrometric detections: an anomaly flag, anomaly score, estimated great-circle axis, and curvature sign. FGOC operates without orbit fitting, requires no modification to LSST AP or MOPS logic, and runs in less than one millisecond per short arc. It provides early anomaly detection, improved short-arc linking, and a complementary diagnostic layer for evaluating astrometric stability during LSST commissioning and early operations.