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/special_tiles_manager.gd

@@ -558,11 +558,7 @@ func spawn_powerups_around(center: Vector2i, force_powerups: bool = true, only_c
 						item_id = rng.randi_range(7, 10)
 					else:
 						# 20% Chance for PowerUp
-						if LobbyManager.is_game_mode(GameMode.Mode.TEKTON_DOORS):
-							# Restrict to Speed (11) and Ghost (14) for Tekton Doors
-							item_id = [11, 14].pick_random()
-						else:
-							item_id = rng.randi_range(11, 14)
+					item_id = rng.randi_range(11, 14)
 				
 				var cell = Vector3i(pos.x, 1, pos.y)