M1: enlarge arena to 150x150m + container-wall ring

- Ground scaled to 150x150m; dirt re-tiled to 24x (~6.25m/tile) to hold look
- ArenaWalls: 4 invisible BoxCollider physics walls (4m tall, sealed corners)
- Junkyard_Walls: 96 cargo containers (24/side) ringing the perimeter as the
  visible wall (Junkyard models pack; physics handled by the invisible walls)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Assets/Materials/Arena_Ground_Dirt.mat

@@ -27,7 +27,7 @@ Material:
     m_TexEnvs:
     - _BaseMap:
         m_Texture: {fileID: 2800000, guid: f24140bb54047334da16c579c56d43b3, type: 3}
-        m_Scale: {x: 8, y: 8}
+        m_Scale: {x: 24, y: 24}
         m_Offset: {x: 0, y: 0}
     - _BumpMap:
         m_Texture: {fileID: 0}
@@ -51,7 +51,7 @@ Material:
         m_Offset: {x: 0, y: 0}
     - _MainTex:
         m_Texture: {fileID: 2800000, guid: f24140bb54047334da16c579c56d43b3, type: 3}
-        m_Scale: {x: 16, y: 16}
+        m_Scale: {x: 24, y: 24}
         m_Offset: {x: 0, y: 0}
     - _MetallicGlossMap:
         m_Texture: {fileID: 0}