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/camera_context_manager.gd

@@ -52,9 +52,6 @@ func _calculate_target_position() -> Vector3:
 		bounds = bounds_gauntlet
 	elif mode == GameMode.Mode.STOP_N_GO:
 		bounds = bounds_stop_n_go
-	elif mode == GameMode.Mode.TEKTON_DOORS:
-		bounds = bounds_doors
-		target_y = 32.3 # Doors uses a higher overlook
 
 	# Clamp X and Z
 	target_x = clamp(target_x, bounds.min_x, bounds.max_x)