feat(terminal): support external byte transports for terminal sessions

The terminal store and widget are transport-agnostic: output enters through Session.feedOutput and input leaves through a small PtyGateway. Only UiApp's installation of that gateway tied the element to local PTYs.

Add an external transport for application-owned byte streams such as SSH, serial, or a websocket. External keys feed the same ghostty-vt session and are routed to the app's transport for writes and resizes; local PTY keys stay on the effects path unchanged. Expose feed and transport methods on UiApp so the app can drive the session from its own worker.