backend: move NetworkBackend and ABackend into backend/ folder

- network_backend.py -> backend/network.py
- a_backend.py -> backend/a_backend.py
- update all imports accordingly

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

transceiver.py

@@ -1,7 +1,7 @@
 from a_transceiver import ATransceiver
 from msg.container import MsgContainer
 from msg.talker import MsgTalker
-from a_backend import ABackend
+from backend.a_backend import ABackend
 
 
 class Transceiver(ATransceiver, MsgTalker):