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/Viewer/LtsDataView/lts_data_view.cpp

@@ -21,20 +21,20 @@
 #include <LtsWidgets/lts_message_dialog.h>
 #include <LtsUtilsSettings/settings.h>
 
-#include <QClipboard>
-#include <QComboBox>
-#include <QFile>
-#include <QFileDialog>
-#include <QKeyEvent>
-#include <QLineEdit>
-#include <QPushButton>
-#include <QStyledItemDelegate>
-#include <QTimer>
+#include <QtGui/QClipboard>
+#include <QtWidgets/QComboBox>
+#include <QtCore/QFile>
+#include <QtWidgets/QFileDialog>
+#include <QtGui/QKeyEvent>
+#include <QtWidgets/QLineEdit>
+#include <QtWidgets/QPushButton>
+#include <QtWidgets/QStyledItemDelegate>
+#include <QtCore/QTimer>
 #include <QScroller>
-#include <QJsonObject>
-#include <QJsonArray>
-#include <QJsonDocument>
-#include <QDebug>
+#include <QtCore/QJsonObject>
+#include <QtCore/QJsonArray>
+#include <QtCore/QJsonDocument>
+#include <QtCore/QDebug>
 
 /// get the list of visual index as they are display
 namespace LtsDataView