refactor: extract Cart.build_state/1 as single source of truth for cart state

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

lib/simpleshop_theme/cart.ex

@@ -166,6 +166,23 @@ defmodule SimpleshopTheme.Cart do
   # Helpers
   # =============================================================================
 
+  @doc """
+  Builds the full display state for a cart.
+
+  Takes raw cart items (list of {variant_id, quantity} tuples) and returns
+  a map with hydrated items, count, and formatted subtotal. Single source
+  of truth for cart state computation — used by CartHook.
+  """
+  def build_state(raw_cart) do
+    hydrated = hydrate(raw_cart)
+
+    %{
+      items: hydrated,
+      count: item_count(raw_cart),
+      subtotal: format_subtotal(hydrated)
+    }
+  end
+
   @doc """
   Returns the total item count in the cart.
   """