Move packed GUID reading into Packet class alongside readUInt8/readFloat.
Replace 121 UpdateObjectParser::readPackedGuid(packet) calls with
packet.readPackedGuid() across 4 files, reducing coupling between
Packet and UpdateObjectParser.
Add Packet::hasFullPackedGuid() method and remove identical standalone
definitions from game_handler.cpp and packet_parsers_classic.cpp.
Replace 53 free-function calls with method calls.
Add getRemainingSize() one-liner to Packet class and replace all 656
instances of getSize()-getReadPos() across game_handler, world_packets,
and both packet parser files.
