Add ValidateConfigUpdate to StreamManager with immutability rules for storage type,
mirror, sources, and retention policy; sealed stream guard; MaxConsumers decrease
prevention; even-replica rejection; and subject overlap detection against peer streams.
Wire the check into CreateOrUpdate for all update paths. 12 new tests in
ConfigUpdateValidationTests.cs cover all rules including the StreamManager integration test.
Joseph Doherty
5e49006cfa