refactor(byteframe): replace read-overflow panic with sticky error

ByteFrame previously panicked on out-of-bounds reads, which crashed
the server when parsing malformed client packets. Now sets a sticky
error (checked via Err()) and returns zero values, matching the
encoding/binary scanner pattern. The session recv loop checks Err()
after parsing to reject malformed packets gracefully.

Also replaces remaining panic("Not implemented") stubs in network
packet Build/Parse methods with proper error returns.

server/channelserver/sys_session.go

@@ -256,6 +256,14 @@ func (s *Session) handlePacketGroup(pktGroup []byte) {
 		)
 		return
 	}
+	if bf.Err() != nil {
+		s.logger.Warn("Malformed packet (read overflow during parse)",
+			zap.String("name", s.Name),
+			zap.Stringer("opcode", opcode),
+			zap.Error(bf.Err()),
+		)
+		return
+	}
 	// Handle the packet.
 	handler, ok := s.server.handlerTable[opcode]
 	if !ok {