experimental: remove Tekton Doors entirely

- Delete portal_mode_manager.gd, portal_door.gd, portal_door.tscn
- Strip all Tekton Doors logic from main.gd, player.gd, lobby.gd,
  lobby_room.gd, lobby_manager.gd, camera_context_manager.gd,
  music_manager.gd, tekton.gd, enhanced_gridmap.gd,
  playerboard_manager.gd, special_tiles_manager.gd
- Remove TK enum (TEKTON_DOORS=2), mode_config schema, arena area
- Update tests: 3 modes instead of 4
- Strip HowToPlay tab from main.tscn

scripts/managers/playerboard_manager.gd

@@ -230,10 +230,6 @@ func _check_and_refill_grid_if_needed(server_gridmap: Node):
 			break
 			
 	if not has_items:
-		if LobbyManager.is_game_mode(GameMode.Mode.TEKTON_DOORS):
-			# Tekton Doors handles its own wall-aware refill in PortalModeManager
-			return
-			
 		print("[PlayerboardManager] Floor 1 empty! Respawning tiles with Scarcity...")
 		# Call randomize_floor on floor 1 using ScarcityController
 		# ScarcityController is a global class, so we can pass its static function as a Callable
@@ -372,18 +368,7 @@ func auto_put_item() -> bool:
 			var pos = neighbor.position
 			var cell_3d = Vector3i(pos.x, 1, pos.y)
 			if enhanced_gridmap.get_cell_item(cell_3d) == -1 and not player.is_position_occupied(pos):
-				# TEKTON DOORS: Avoid portal doors
-				var is_on_portal = false
-				if LobbyManager.is_game_mode(GameMode.Mode.TEKTON_DOORS):
-					var doors = get_tree().get_nodes_in_group("PortalDoors")
-					for door in doors:
-						var door_grid = enhanced_gridmap.local_to_map(enhanced_gridmap.to_local(door.global_position))
-						if Vector2i(door_grid.x, door_grid.z) == pos:
-							is_on_portal = true
-							break
-				
-				if not is_on_portal:
-					valid_put_positions.append(pos)
+				valid_put_positions.append(pos)
 
 	if valid_put_positions.is_empty():
 		return false