PC3 Phase 1.1: game boots clean from a bare clone

Root cause: a stale .godot/global_script_class_cache.cfg was force-committed
to the repo (last regenerated before the avatar system merged), and Godot
does not rebuild the global class registry on a plain headless scene run --
only an editor scan / `--import` does. So every cross-file `class_name`
reference in the avatar module failed to resolve from a fresh clone, and the
AvatarSystem autoload failed to instantiate entirely.

- Untrack .godot/ and sandbox_probe/.godot/ (already in .gitignore; were
  force-added during old WIP checkpoints)
- Reference sibling avatar scripts via preload() constants instead of their
  global class_name, so the module parses independent of scan order. Keeps
  the class_name declarations for editor/tooling use.
- avatar_loadout.gd's static from_json() factory self-references via a
  preload const too (a script can't resolve its own class_name mid-compile
  on a cold cache).
- smoke_test_avatar_system.gd: same preload treatment so it runs from a bare
  `--script` invocation.

Verified from a wiped .godot/: main_menu.tscn and game.tscn boot with zero
script errors; smoke_test_avatar_system 10/10, smoke_test_player 15/15,
smoke_test_main_menu_ui 13/13, smoke_test_test_level PASS.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015jtxAiu5dAW4WxCNdGkrkJ

scripts/avatar/avatar_loadout.gd

@@ -8,6 +8,15 @@
 class_name AvatarLoadout
 extends RefCounted
 
+# Sibling avatar scripts are referenced via preload rather than by their
+# global class_name so this module parses correctly from a bare clone (a
+# headless `godot <scene>` run does not rebuild the global class registry;
+# only an editor scan / `--import` does). See docs/architecture/avatar.md.
+const AvatarSlotRegistry := preload("res://scripts/avatar/avatar_slot_registry.gd")
+# Self-reference for the static factory below -- referring to our own
+# class_name doesn't resolve during a cold-cache compile (see note above).
+const _Self := preload("res://scripts/avatar/avatar_loadout.gd")
+
 
 ## Maximum number of accessories allowed in the accessory slot.
 const MAX_ACCESSORIES: int = 8
@@ -58,7 +67,7 @@ func _init() -> void:
 
 ## Create a loadout from a JSON string.
 static func from_json(json_string: String) -> AvatarLoadout:
-	var loadout := AvatarLoadout.new()
+	var loadout := _Self.new()
 	var data: Variant = JSON.parse_string(json_string)
 	if not (data is Dictionary):
 		loadout.validation_errors.append("Loadout JSON root is not an object")