- Add network/packetid_test.go with tests for PacketID type, constants,
String() method, ranges, uniqueness, and sequential verification
- Add network/mhfpacket/msg_sys_core_test.go with round-trip tests for
MsgSysAck (including large payloads), MsgSysNop, and MsgSysEnd
- Expand common/stringstack/stringstack_test.go with Lock/Unlock tests
achieving 100% coverage