step-5: unify ws handler (device_ws.py, chat.py)

- device_ws.py: dispatch home_request/popup_request to HomeFormatter/PopupFormatter
  via async tasks; each request gets a UUID request_id for frame correlation
- chat.py: remove chat_stream WS endpoint (superseded by unified device WS);
  keep POST /chat REST fallback unchanged
- 5 new integration tests pass; all 22 existing device_ws tests still pass

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

V3_MIGRATION_PLAN.md

@@ -253,7 +253,7 @@ pytest tests/test_ws_unified.py
 ```
 
 **Status**:
-- [ ] Step 5 complete
+- [x] Step 5 complete
 
 **Commit**: After tests pass, commit with:
 ```