feat: let a cart line hold 500 at most, and judge that with the minimum order

[JasonYv]

What this is

The owner set the most of one variant a cart line may hold: 500. It was 10,000, a guard against scripted abuse carried over from the previous implementation (design §11, decision 9).

At 500 it is a rule of the shop that a buyer will meet, so it is judged where the minimum order is — quantityIssue, which add-to-cart and cart pricing share. A line above it that is already in a cart cannot be ordered either, and says the figure. A quantity field carries it as max, on a product page and in the cart. The admin refuses a minimum order above it.

How it was verified

Check	Result
lint, type check, build, both smoke runs	Pass
Tests inside workerd	638 of 638, five of them new
Red and green	12 deliberate breakages, each one noticed: the boundary both ways, the figure itself, the order in which the ceiling and the stock are said, the max on both pages

The sample's largest minimum order is 200, so nothing in it is affected.