WIP checkpoint: fix script-attach typo, missing class_name, var name mismatch

- scenes/avatar_customization.tscn: script_extense -> script (typo meant
  the customization UI script was never actually attached to the scene)
- scripts/avatar/avatar_loadout.gd: add missing class_name AvatarLoadout
  (avatar_system.gd references the type everywhere; only this file among
  the avatar scripts lacked its class_name)
- scripts/main_menu.gd: _ready() referenced _customize_button/_settings_button/
  _quit_button but the @onready vars are named without the leading
  underscore

scripts/avatar/avatar_loadout.gd

@@ -5,6 +5,7 @@
 ##
 ## See docs/architecture/avatar.md §5 for the loadout JSON format.
 
+class_name AvatarLoadout
 extends RefCounted
 
 

scripts/main_menu.gd

@@ -40,9 +40,9 @@ signal quit_requested
 # ---------------------------------------------------------------------------
 
 func _ready() -> void:
-	_customize_button.pressed.connect(_on_customize_pressed)
-	_settings_button.pressed.connect(_on_settings_pressed)
-	_quit_button.pressed.connect(_on_quit_pressed)
+	customize_button.pressed.connect(_on_customize_pressed)
+	settings_button.pressed.connect(_on_settings_pressed)
+	quit_button.pressed.connect(_on_quit_pressed)
 
 
 # ---------------------------------------------------------------------------