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_parse_coverage_test.go

@@ -298,7 +298,7 @@ func TestBuildCoverage_Implemented(t *testing.T) {
 		defer func() {
 			_ = recover() // may panic due to bug
 		}()
-		pkt := &MsgSysLogout{Unk0: 1}
+		pkt := &MsgSysLogout{LogoutType: 1}
 		bf := byteframe.NewByteFrame()
 		_ = pkt.Build(bf, ctx)
 	})