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/game_mode.gd

@@ -1,12 +1,7 @@
 extends RefCounted
 class_name GameMode
 
-enum Mode {
-	FREEMODE = 0,
-	STOP_N_GO = 1,
-	TEKTON_DOORS = 2,
-	GAUNTLET = 3
-}
+enum Mode { FREEMODE = 0, STOP_N_GO = 1, GAUNTLET = 2 }
 
 static func from_string(mode: String) -> Mode:
 	match mode:
@@ -14,12 +9,9 @@ static func from_string(mode: String) -> Mode:
 			return Mode.FREEMODE
 		"Stop n Go":
 			return Mode.STOP_N_GO
-		"Tekton Doors":
-			return Mode.TEKTON_DOORS
 		"Candy Pump Survival":
 			return Mode.GAUNTLET
-		_:
-			return Mode.FREEMODE
+	return Mode.FREEMODE
 
 static func mode_to_string(mode: Mode) -> String:
 	match mode:
@@ -27,15 +19,13 @@ static func mode_to_string(mode: Mode) -> String:
 			return "Freemode"
 		Mode.STOP_N_GO:
 			return "Stop n Go"
-		Mode.TEKTON_DOORS:
-			return "Tekton Doors"
 		Mode.GAUNTLET:
 			return "Candy Pump Survival"
 		_:
 			return "Freemode"
 
 static func is_restricted(mode: Mode) -> bool:
-	return mode == Mode.STOP_N_GO or mode == Mode.TEKTON_DOORS or mode == Mode.GAUNTLET
+	return mode == Mode.STOP_N_GO or mode == Mode.GAUNTLET
 
 static func get_all_modes() -> Array[String]:
-	return ["Freemode", "Stop n Go", "Tekton Doors", "Candy Pump Survival"]
+	return ["Freemode", "Stop n Go", "Candy Pump Survival"]