Red Arrow: Your actual movement (delta_x, delta_y combined)
Green Arrow: The direction you want to measure progress in
Blue Arrow: The corrected distance that counts toward your goal
Why cos(angle)? It finds how much of your movement is "useful" in the measurement direction!
0°: Moving directly toward goal → cos(0°) = 1 → Full distance counts
45°: Moving diagonally → cos(45°) = 0.71 → 71% of distance counts
90°: Moving sideways → cos(90°) = 0 → No progress toward goal!