Function build_occupancy_grid 

pub fn build_occupancy_grid(
    link_lengths: &[f64; 2],
    obstacles: &[CircleObstacle],
    grid_size: usize,
) -> JointSpaceGrid

Builds the joint-space occupancy grid for a 2-joint arm.

Iterates over all discretized joint angle combinations, computes the arm configuration, and checks for collision with each obstacle.