Fix one instruction so the robot reaches the target

A robot starts on S in a grid with obstacles, facing up, and runs a program of commands: F (forward one cell), B (back one cell, same facing), L and R (turn in place). You must change exactly one command to a different one. Return whether some change makes the robot end on T without ever leaving the grid or hitting an obstacle. Brute force tries every change; precompute the state before each position to make it faster.