Improved the science accuracy of grazing transits in ExoSim. Previously, the simulator estimated partial transits using a simple straight-line calculation. I replaced that with the actual overlapping-circles formula, treating the visible star and planet as two disks. The simulator now calculates exactly how much of the planet overlaps the star during a grazing transit. The transit percentage and light-curve dip change nonlinearly as the orbital inclination moves from a full transit to a missed one. I also separated the calculation into its own TypeScript utility file so the science logic is easier to understand and maintain.