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

addons/enhanced_gridmap/enhanced_gridmap.gd

@@ -55,8 +55,7 @@ func _ready():
 			# Safety check: Don't auto-randomize if game mode manages its own arena
 			if not (ResourceLoader.exists("res://scripts/managers/lobby_manager.gd") \
 				and get_node_or_null("/root/LobbyManager") \
-				and (get_node("/root/LobbyManager").game_mode == "Stop n Go" \
-				or get_node("/root/LobbyManager").game_mode == "Tekton Doors")):
+				and (get_node("/root/LobbyManager").game_mode == "Stop n Go")):
 				randomize_grid()
 	validate_item_indices()