SharedInputStream class

Shared broadcast input stream wrapper.

Dart's stdin is a single-subscription stream: once listened to, it cannot be listened to again (even if cancelled). This wrapper fans out input into a broadcast stream so TUIs can temporarily stop/restart input listening (e.g. suspend, exec) without triggering "Bad state: Stream has already been listened to".

Important: By design, the underlying source subscription stays alive once started. This enables re-listening within the same process, but it can also keep the Dart event loop alive on real TTYs. Call shutdownSharedStdinStream when the process should be allowed to exit cleanly.