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/music_manager.gd

@@ -47,7 +47,7 @@ func start_music():
 	match game_mode:
 		"Stop n Go":
 			track_path = "res://assets/sounds/stop_n_go.wav"
-		"Freemode", "Tekton Doors", _:
+		"Freemode", _:
 			track_path = "res://assets/sounds/level_bridge.wav"
 	
 	play_track(track_path)