Mixed cart restriction across preorder offers

Current behavior: "Don't allow mixed cart orders" works per offer. If a merchant has two separate preorder offers, products from those offers can't be added to the same cart, even though both are preorders.

Requested behavior: Add an option so the restriction only blocks preorder and regular (in-stock) products from being mixed. Preorder items from different offers should be allowed in the same cart.

Why it matters: Merchants often split offers by variant or size because each one needs its own quantity limits, batches or delivery dates. Moving everything into one offer isn't an option for them, so right now customers can't buy two sizes of the same preorder product in one order.

Possible approach: A setting under the mixed cart option, such as "Block preorder + in-stock only" vs "Block all mixing across offers", so existing merchants keep the current behavior by default.