Class InAppChannel

The channel that reaches the tab someone already has open.

This is the half that makes a notification module worth having in this framework rather than a mail client wrapped in a service: one notify() reaches the mailbox, the phone and the open screen, and the application wires none of the three together.

It broadcasts through @stone-js/realtime, duck-typed from the container, so this package neither imports it nor requires it. The client side is already written: whoever is subscribed to the recipient's channel receives a notification event, with the same rendered message every other channel got.

It reports unreachable rather than failed when nobody is listening, because that is not an error: a person who is not looking at the screen is exactly why the other channels exist.