add frame logging and extract entry point to main.py

- msg/frame_logger.py: MsgListener that writes raw byte stream per
  source to timestamped log files (<YYYY_MM_DD_HH_MM_SS_source>.log)
- main.py: entry point extracted from network_backend.py; adds
  --log-dir argument (default ./log) and wires FrameLogger per transceiver
- network_backend.py: now contains only the NetworkBackend class
- .gitignore: exclude log/ directory

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

msg/frame_logger.py

@@ -0,0 +1,23 @@
+import os
+from datetime import datetime
+from msg.listener import MsgListener
+from msg.container import MsgContainer
+
+
+class FrameLogger(MsgListener):
+    def __init__(self, source_id: str, log_dir: str = './log'):
+        super().__init__()
+        os.makedirs(log_dir, exist_ok=True)
+        ts = datetime.now().strftime('%Y_%m_%d_%H_%M_%S')
+        filename = f"{ts}_{source_id.strip()}.log"
+        self._file = open(os.path.join(log_dir, filename), 'wb')
+
+    def is_msg(self, msg: MsgContainer) -> bool:
+        return True
+
+    def on_recv(self, msg: MsgContainer):
+        self._file.write(msg.data)
+        self._file.flush()
+
+    def close(self):
+        self._file.close()