Background session runner gains an onMaxLifetime lifecycle hook

The child-process session manager (mr/Sr) constructor now accepts onMaxLifetime and onTerminating callbacks, and the created instance is stored on a ref (St.current) so other code can reach the active runner session, e.g. to check terminationRequested. The background-session runner's max-lifetime timeout now calls the onMaxLifetime hook to decide whether to wait or kill the child process, instead of always killing unless a turn is in flight; hook errors fall back to the previous default behavior. The background-result follow-up clear callback also now reports whether termination was requested (B?.(!1, k || St.current?.terminationRequested === !0)).