Qt6 migration: Fix BUILD_RTI=OFF build errors in TrainingAreaBase

- training_area_telegrams.cpp: Fixed QJsonObject initializer syntax (Qt6 requires {{}} not ({}))
- training_area_telegrams.cpp: Fixed fromJson() methods to access m_* members directly
- training_area_telegrams.cpp: Fixed brace mismatches in multiple toJson() methods
- training_area_telegrams.h: Changed Header struct to public members for consistency
- tx_mixin.cpp: Fixed getEquipmentNumber() template for direct member access
- lts_coordinate.cpp: Added missing #include <iomanip>
- MIGRATION_STATUS.md: Added migration progress tracking document

C++17 constraint enforced due to RTI Connext DDS 7.7.0 incompatibility with C++20+

Build status: BUILD_RTI=OFF successful (0 errors, warnings only)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

qt6-after/qt6-migrated-legatus/Lts/Plugins/Messaging/Gui/Messaging/msg_messaging.cpp

@@ -10,7 +10,7 @@
 #include <LtsSession/lts_session_data.h>
 
 #include <LtsPrinting/printing.h>
-#include <QTreeView>
+#include <QtWidgets/QTreeView>
 
 namespace Messaging
 {

qt6-after/qt6-migrated-legatus/Lts/Plugins/Messaging/Gui/Messaging/msg_messaging_gui_plugin.cpp

@@ -17,7 +17,7 @@
 #include <Plugins/OperatorManagement/opm_data_plugin_interface.h>
 #include <Services/Messaging/msg_operator_rights.h>
 
-#include <QTimer>
+#include <QtCore/QTimer>
 
 
 namespace Messaging

qt6-after/qt6-migrated-legatus/Lts/Plugins/Messaging/Gui/Messaging/msg_new_message_dialog.h

@@ -13,7 +13,7 @@
 
 #include <Plugins/OperatorManagement/opm_gui_combobox_plugin_interface.h>
 
-#include <QAbstractListModel>
+#include <QtCore/QAbstractListModel>
 
 namespace Ui
 {

qt6-after/qt6-migrated-legatus/Lts/Plugins/Messaging/Gui/Messaging/msg_show_message_dialog.cpp

@@ -10,9 +10,9 @@
 #include <LtsSession/lts_session_data.h>
 #include <LtsPrinting/printing.h>
 
-#include <QShowEvent>
-#include <QDateTime>
-#include <QTimer>
+#include <QtGui/QShowEvent>
+#include <QtCore/QDateTime>
+#include <QtCore/QTimer>
 
 
 namespace Messaging