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A self-contained add-to-cart form. Posts the line via bbk-add-to-cart-controller
(fetch) and hands the rendered cart body to the drawer over the
`bbk-cart:changed` window event.
Props:
purchasable ProductVariant id. Omit to render no hidden id field — the host
must then supply [data-bbk-purchasable-input] itself (e.g. a
variant picker writing the selected id into it).
quantity Integer for the hidden quantity field, or false to omit it
(the host then puts its own name="quantity" control in the slot).
The button and any quantity control come from the slot, so the host owns all
appearance. Extra attributes (class, etc.) land on the