fix: retry connection on network error instead of permanently dropping it

error_received() no longer sets _dead=True; it just closes the transport
so the existing retry loop in heartbeat_sender (hbc) and sendto (hbc_mini)
reopens the connection on the next interval. This allows hbc to recover
when it starts before network connectivity is established.

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

hbd/client/main.py

@@ -172,9 +172,8 @@ class HeartbeatProtocol(asyncio.DatagramProtocol):
             self.logger.error(f"Error processing datagram: {e}", exc_info=True)
     
     def error_received(self, exc):
-        """Handle protocol errors."""
-        self.logger.warning(f"Protocol error on {self.connection.addr}: {exc} — dropping connection")
-        self.connection._dead = True
+        """Handle protocol errors — close transport so the heartbeat sender retries."""
+        self.logger.warning(f"Protocol error on {self.connection.addr}: {exc} — will retry")
         self.connection.close()