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_small_test.go

@@ -123,8 +123,8 @@ func TestParseSmallLogout(t *testing.T) {
 			if err != nil {
 				t.Fatalf("Parse() error = %v", err)
 			}
-			if pkt.Unk0 != tt.unk0 {
-				t.Errorf("Unk0 = %d, want %d", pkt.Unk0, tt.unk0)
+			if pkt.LogoutType != tt.unk0 {
+				t.Errorf("Unk0 = %d, want %d", pkt.LogoutType, tt.unk0)
 			}
 		})
 	}