Scoped values (Java 25): ScopedValue.where(...).run(...) as an alternative to ThreadLocal

Applications often need to pass contextual data, such as the current user or a request ID, down through many method calls without adding a parameter to every method. The traditional tool is ThreadLocal, a variable with a separate value per thread. It has drawbacks: any code can call set at any time, so data flow is hard to follow; values live until explicitly removed, which leaks data and memory in thread pools; and child threads that inherit values must copy them. With huge numbers of virtual threads these costs grow.