feat: Remove address cloning

[phorward]

Removes the address cloning mechanism that was previously used to snapshot
shipping addresses during order freeze. Instead, address data is now persisted
directly via relational bone configuration.

Before: When freezing an order/cart, clone_address() created a full copy
of the address record and stored a cloned_from back-reference.

After: CartNodeSkel.shipping_address (and the corresponding bone in
OrderSkel) uses refKeys="*" with updateLevel=UpdateLevel.OnValueAssignment,
capturing the full address at assignment time. Additionally, shipping_address
is added to frozen_values on CartNodeSkel to prevent later mutation.

Changes

• Remove clone_address() method and all call sites in cart.py / order.py
• Remove cloned_from RelationalBone from AddressSkel
• Set refKeys="*" and updateLevel=UpdateLevel.OnValueAssignment on shipping
  address bones in cart/order skeletons
• Add shipping_address to CartNodeSkel.frozen_values
• Drop now-unused shop_address Datastore index on cloned_from
• Remove viur.shop.skeleton.address.cloned_from translation keys

[sveneberth]

Maybe write address to frozen_values