WIP checkpoint: fix static registry, uint32 type, GLTF API calls, ARRAY_BONES, missing slot handler

- avatar_slot_registry.gd: _slot_data/_slot_definitions made static (every
  caller uses AvatarSlotRegistry.method() on the class, never an instance,
  so the old instance _init() populating them was never invoked); moved
  registry build to static func _static_init()
- avatar_catalog.gd: compute_loadout_hash() return type uint32 -> int
  (GDScript has no fixed-width uint type)
- avatar_loader.gd: GLTFDocument.parse() (doesn't exist) -> the real
  append_from_buffer()+generate_scene() API; Error.parse_error() (doesn't
  exist) -> error_string(); Mesh.ARRAY_JOINTS (doesn't exist) ->
  Mesh.ARRAY_BONES; removed a dead 'mesh is MultiMesh' check that can
  never be true (MeshInstance3D.mesh is always Mesh, never MultiMesh);
  fixed _merge_node_into_avatar to recurse into a Skeleton3D's children
  (skinned meshes are commonly children of the skeleton in a GLB export,
  the old code returned before ever reaching them)
- avatar_customization_ui.gd: added the missing _on_slot_button_pressed
  handler (referenced by a button connect, never defined -- just calls
  the existing _select_slot())

Verified: godot --headless --import + --quit is fully clean, and the
avatar_customization.tscn scene now instantiates successfully.

Remaining, scoped separately: avatar_visualizer.gd calls a
AvatarLoader.load_slot() static async API and SlotItem/SlotResult types
that don't exist -- AvatarLoader's real API (matching the project's own
avatar.md spec: one canonical skeleton, attachments retargeted onto it)
is instance-based load_base_body()/load_attachment(). AvatarVisualizer
needs rewriting to use that real API; it's the one out of step with spec,
not AvatarLoader.

scripts/avatar/avatar_customization_ui.gd

@@ -326,6 +326,11 @@ func _populate_items() -> void:
 # Slot selection
 # ---------------------------------------------------------------------------
 
+## Handle a slot button press.
+func _on_slot_button_pressed(slot_id: String) -> void:
+	_select_slot(slot_id)
+
+
 ## Select a slot and refresh the item list.
 func _select_slot(slot_id: String) -> void:
 	_selected_slot = slot_id