Sale product brand exception

This module extends product brands with a new "Sale" setting and uses the sale exception framework to control what can be sold together.

On each product brand form you now find a Sale section with the option "sale_order_mixed", described as whether a sale order can contain products from different brands.

A ready-made exception rule named "Product brands mixed" is installed on sale orders. Its description states that products from different brands cannot be mixed in one order. When a sale order contains lines whose products belong to more than one brand, and not all of those brands allow mixing, the rule is triggered and the order is flagged as an exception, just like other sale exceptions.

• The rule is active by default when the module is installed.
• In demo data the rule is switched off so it does not interfere with other tests.
• The check looks at the brands of the products on the order lines; if only one brand is present, nothing happens.
• If every brand involved has the mixing option enabled, the order is not flagged.

The module touches product.brand (new field and form view) and sale.order (the check method used by the exception rule). It does not add its own menus; the brand option appears inside the existing product brand form, and the exception appears wherever sale exceptions are shown.