Separate JBD decoding from serial transport

Move response decoding into jbd_proto and the data container into jbd_snapshot. Preserve the existing jbd_bms imports and test the transport boundary.

tests/test_jbd_bms.py

@@ -1,13 +1,15 @@
 import pytest
 from unittest.mock import MagicMock, patch
-from bmspy.jbd_bms import (
-    JBDBMS,
+from bmspy.jbd_snapshot import JBDBMS
+from bmspy.jbd_proto import (
     bytes_to_digits,
     bytes_to_date,
     convert_to_signed,
     verify_checksum,
     parse_03_response,
     parse_04_response,
+)
+from bmspy.jbd_bms import (
     requestMessage,
     serial_cleanup,
     collect_data,