feat(items): add HoldableItem with hand slot detection, fix detach_item position

- Character registers in "characters" group on _ready for group scanning
- detach_item saves/restores global_position after reparenting
- New HoldableItem base class: scans "characters" group on drag_released,
  attaches to nearest free hand within 60px radius, detaches on pickup

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/characters/character.gd

@@ -32,6 +32,7 @@ func _ready() -> void:
 	if data != null:
 		_update_visual_state()
 	_refresh_outfit_layers()
+	add_to_group("characters")
 
 
 func set_state(new_state: CharacterData.State) -> void:
@@ -110,10 +111,12 @@ func detach_item(hand: String) -> Node2D:
 	if slot == null or slot.get_child_count() == 0:
 		return null
 	var item: Node2D = slot.get_child(0) as Node2D
+	var saved_pos: Vector2 = item.global_position
 	slot.remove_child(item)
 	var scene_parent: Node = get_parent()
 	if scene_parent != null:
 		scene_parent.add_child(item)
+	item.global_position = saved_pos
 	return item