feat(game): add persistent bush-wall breakthroughs

MVP.md

@@ -109,8 +109,10 @@ approximately five minutes. There is no match time limit.
   simultaneous turns.
 - A small server-enforced minimum interval exists between accepted moves. Its
   value is configurable for playtesting.
-- Rejected movement into a wall does not change position and is not a
-  successful move.
+- A server-accepted impact against the same closed interior wall from the same
+  cell and direction is tracked per player. The first four impacts do not move
+  the player; the fifth within three seconds of each previous impact opens the
+  passage in both cells and completes that move. Perimeter walls never open.
 - Players do not block each other and may occupy the same cell.
 - A meeting occurs when both players occupy the same cell.
 - There are no attacks, health, damage, or other combat mechanics in MVP.