Function dijkstra_plan 

pub fn dijkstra_plan(
    obstacle_map: &Map,
    start: (usize, usize),
    goal: (usize, usize),
) -> Option<Vec<(usize, usize)>>

Run Dijkstra’s algorithm on a grid obstacle map.

Returns the shortest path from start to goal as a list of (row, col) coordinates, or None if no path exists.