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

scenes/main.tscn

@@ -2249,25 +2249,6 @@ text = "[b]Stop n Go[/b]
 - Your objective is to reach the mission tiles at the far end of the arena and safely carry them back to your starting zone.
 - The first player to complete 8 missions and reach the finish floor wins."
 
-[node name="Tekton Doors" type="MarginContainer" parent="HowToPlayPanel/Panel/VBox/TabContainer" unique_id=123456799]
-visible = false
-layout_mode = 2
-theme_override_constants/margin_left = 10
-theme_override_constants/margin_top = 10
-theme_override_constants/margin_right = 10
-theme_override_constants/margin_bottom = 10
-metadata/_tab_index = 2
-
-[node name="RichTextLabel" type="RichTextLabel" parent="HowToPlayPanel/Panel/VBox/TabContainer/Tekton Doors" unique_id=123456800]
-layout_mode = 2
-bbcode_enabled = true
-text = "[b]Tekton Doors[/b]
-
-- Navigate a sprawling arena connected by color-coded portal doors.
-- Grab tiles and match goal patterns to earn mission completions.
-- Use doors to quickly teleport across rooms, but watch out for closures and traps.
-- The first player to complete 8 missions and reach the finish room wins."
-
 [node name="Controls" type="MarginContainer" parent="HowToPlayPanel/Panel/VBox/TabContainer" unique_id=123456805]
 visible = false
 layout_mode = 2