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>

scripts/hbc_mini.py

@@ -797,8 +797,7 @@ class _HeartbeatProtocol(asyncio.DatagramProtocol):
             self._log.error("datagram error: %s", e)
 
     def error_received(self, exc):
-        self._log.warning("protocol error on %s: %s — dropping connection", self._conn.addr, exc)
-        self._conn._dead = True
+        self._log.warning("protocol error on %s: %s — will retry", self._conn.addr, exc)
         self._conn.close()