Gateway restart notices: explicit status{restarting} signal, correct timing

The app previously showed 'Gateway restarting' on every connection loss.
Now the gateway broadcasts status{state=restarting} on its shutdown path
(before closing the sockets), and the app:

- posts 'Gateway restarting' immediately on that frame (not on the
  socket-drop transition, which lags by the ~20s WS ping timeout)
- posts 'Gateway online' on the next reconnect only when the restart
  notice was posted (latch) - a plain network drop shows neither, just
  the reconnect banner
- drops the 'Gateway is restarting...' banner (replaced by the chat notice)

Docs (04-wire-protocol, frames.schema.json) updated: restarting is no
longer reserved. Test for the disconnect broadcast added to the local
hermes-agent test mirror (git-ignored, not committed).

gateway-plugin/adapter.py

@@ -1212,6 +1212,9 @@ class AndroidAdapter(BasePlatformAdapter):
 
         # M5: announce gateway health to connected clients (none yet at
         # startup; the frame + plumbing exist for future transitions).
+        # Reset in case this adapter instance previously went down (the
+        # gateway may reconnect the same adapter after a fatal error).
+        self._gateway_status = protocol.STATUS_ONLINE
         await self._ws_server.broadcast(protocol.status(self._gateway_status))
 
         # M3: ensure the default (home) channel exists in the directory so the
@@ -1238,6 +1241,13 @@ class AndroidAdapter(BasePlatformAdapter):
 
     async def disconnect(self) -> None:
         """Tear down the platform: stop the server, close device sockets."""
+        # Tell live clients the gateway is going away (restart/shutdown) so
+        # the app can distinguish a clean gateway teardown from a plain
+        # network drop: the "Gateway restarting" chat notice is shown only
+        # when this frame was received (docs/04 §status).
+        self._gateway_status = protocol.STATUS_RESTARTING
+        with contextlib.suppress(Exception):
+            await self._ws_server.broadcast(protocol.status(self._gateway_status))
         with contextlib.suppress(ImportError):
             from gateway.status import release_scoped_lock