refactor(mhfpacket): rename 15 Unk fields with identified meanings

Replace unknown field names with descriptive names based on handler
logic analysis, switch dispatch patterns, DB query context, and
inline comments:

- ObjectHandleID, IsQuest, ItemIDCount, MaxCount, TokenLength,
  FormatVersion, LogoutType (high confidence from comments/constants)
- QueryType, DataType, MissionIndex, CheckOnly, RequestType,
  ExchangeType, TournamentID (confirmed by handler switch/if usage)

Also fix MsgSysLogout.Build calling ReadUint8 instead of WriteUint8.

network/mhfpacket/msg_mhf_register_event.go

@@ -12,7 +12,7 @@ type MsgMhfRegisterEvent struct {
 	Unk0      uint16
 	WorldID   uint16
 	LandID    uint16
-	Unk1      bool
+	CheckOnly bool
 }
 
 // Opcode returns the ID associated with this packet type.
@@ -26,7 +26,7 @@ func (m *MsgMhfRegisterEvent) Parse(bf *byteframe.ByteFrame, ctx *clientctx.Clie
 	m.Unk0 = bf.ReadUint16()
 	m.WorldID = bf.ReadUint16()
 	m.LandID = bf.ReadUint16()
-	m.Unk1 = bf.ReadBool()
+	m.CheckOnly = bf.ReadBool()
 	bf.ReadUint8() // Zeroed
 	return nil
 }