🧲 Interactive Magnetic Field Line Display (2D): Magnets, Current Loop and Compass

2D magnetic field display: iron-filing grid, RK4-traced field lines, up to three bar magnets, a Biot-Savart current loop, a uniform background field and a damped compass needle, with four in-page numerical checks (Gauss, Biot-Savart, dipole limit, Ampere).