Handles tanks positioned on the river bank. While the probe point (16 pixels ahead in the facing direction) reads solid terrain ($FF), the tank moves normally via tank_move_entry. When the probe reads anything other than $FF (river edge reached), the tank stops moving and fires a shell. While the shell is flying or exploding, the tank returns to the main loop without updating its position, keeping it stationary. The tank never reverses direction. Shell trajectory depends on tank orientation and uses pseudo-random speed.