Checkpoint: relocation

relocate is BFS-lite: seed the start with distance 0, relax neighbors in passes until nothing changes, read the target's distance — or -1 when the fill never reaches it. It fuses navigation, guards, and the sentinel into one method, which is why Q4 sometimes asks for exactly this. If your first version moved into already-shorter cells forever, the relaxation condition was missing dist[nr][nc] > dist[r][c] + 1.