Exclusive end of a subscription: the first group that will never be delivered.

Zero represents an empty track. MOQX follows the deployed Lite05 correction in https://github.com/moq-dev/moq/pull/2333, approved by the maintainer, rather than the inclusive wording in submitted draft-lcurley-moq-lite-05 section 7.12. See ADR-0014 and the shared publisher/subscriber completion logic.