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_mercenary_huntdata.go

@@ -11,7 +11,7 @@ import (
 // MsgMhfMercenaryHuntdata represents the MSG_MHF_MERCENARY_HUNTDATA
 type MsgMhfMercenaryHuntdata struct {
 	AckHandle uint32
-	Unk0      uint8
+	RequestType uint8
 }
 
 // Opcode returns the ID associated with this packet type.
@@ -22,7 +22,7 @@ func (m *MsgMhfMercenaryHuntdata) Opcode() network.PacketID {
 // Parse parses the packet from binary
 func (m *MsgMhfMercenaryHuntdata) Parse(bf *byteframe.ByteFrame, ctx *clientctx.ClientContext) error {
 	m.AckHandle = bf.ReadUint32()
-	m.Unk0 = bf.ReadUint8()
+	m.RequestType = bf.ReadUint8()
 	return nil
 }