Rotting Oranges

Write `oranges_rotting(grid)` where `0` is empty, `1` is fresh and `2` is rotten. Each minute, rot spreads to fresh neighbours. Return the minutes until none are fresh, or `-1` if some can never rot. [[2,1,1],[1,1,0],[0,1,1]] -> 4 The rot spreads from every rotten orange at once, so seed the queue with *all* of them before starting — that is what makes one BFS give the right answer rather than the distance from a single source. Count fresh oranges up front so the unreachable case is a comparison, not a second scan.