Memoise the discounts a cart has already consumed

[kha333n]

Follow-up to #2637, as suggested in review.

Cart::consumedDiscountIds() re-queried the draft order on every call: once when
the discount set is rebuilt, and once per discount while conditions are checked.
That is D+1 single-row lookups on every calculate, paid by every cart including
those that never reach a checkout.

What changes

The result is memoised on the cart instance. Measured on a cart with four active
discounts, for one calculate():

	draft-order lookups	total queries
before	5	47
after	1	43

The trap, and how it is handled

A memo primed before the first order exists is empty, and stays empty. On a
same-request retry CreateOrder would then see nothing consumed, and consume the
discount a second time — exactly the double-consumption #2637 was written to
prevent.

Order creation is the only thing that invalidates the set, because it writes the
breakdown the method reads. So consumedDiscountIds() takes a $fresh flag and
CreateOrder passes it:

$alreadyConsumed = $cart->consumedDiscountIds(fresh: true);

Everything else reads the memo. The parameter defaults to false, so no other
caller changes.

Tests

tests/core/Unit/Actions/Carts/CreateOrderTest.php:

• can not consume a discount twice on one cart instance — two order
  creations on the same cart object, with no reload between them, asserting
  uses stays at 1. The existing #2637 tests reload the cart between attempts,
  so none of them cover this.

  It passes on 1.x today, and it is the guard for this change: with the memo
  added and the fresh read left out, it fails with
  Failed asserting that 2 matches expected 1 — the double consumption back
  again. Verified by writing the naive memo first and watching it fail.