IMU preintegration and propagation are the same thing

IMU preintegration—standard in factor-graph SLAM—is typically treated as separate from conventional IMU propagation, but they're actually the same computation viewed differently. This work unifies them by showing how to wrap existing propagation code to obtain preintegrated measurements and Jacobians, and vice versa. Tested against GTSAM and RK4 implementations, the framework simplifies translating between error-state definitions and validates implementation correctness.