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>

backend/a_backend.py

@@ -0,0 +1,6 @@
+from abc import ABC
+from a_transceiver import ATransceiver
+
+class ABackend(ABC):
+	def send(self, xcr: ATransceiver, data: bytes):
+		pass