Computes the fair distribution of a discount-token budget across a
basket. Each product offers discount levels, each with a stock capacity;
the service assigns units to levels so total token spend stays within the
allowed band while spreading discounts fairly rather than concentrating
them on the highest level. Pure compute — no side effects. Called by the
checkout service over SigV4 (AWS_IAM), the convention for /internal/*
routes whose caller is one of our own services.