docs: complete the CountdownState codec docstring

pydoclint (run as a test here, unlike vgi-rpc where it is a CI step) wants
Args and Returns on the deserialize side.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: rustyconover

vgi/_test_fixtures/table/_common.py

@@ -76,6 +76,12 @@ def serialize_to_bytes(self) -> bytes:
     def deserialize_from_bytes(cls, data: bytes) -> CountdownState:
         """Unpack a payload written by :meth:`serialize_to_bytes`.
 
+        Args:
+            data: The packed bytes, exactly two little-endian int64s.
+
+        Returns:
+            The restored counter pair.
+
         Raises:
             ValueError: The payload is not the expected width, which means it
                 was not written by this codec.