Fix a bug in the neutral unit logic.

paper_tactics/entities/game.py

@@ -112,6 +112,7 @@ def _make_turn(self, cell: Cell, player: Player, opponent: Player) -> None:
             self._rebuild_reachable_set(opponent, player)
         elif cell in self.trenches:
             player.walls.add(cell)
+            opponent.reachable.discard(cell)
         else:
             player.units.add(cell)
         self._rebuild_reachable_set(player, opponent)