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/Common/LtsScriptTimeline/track_header.cpp

@@ -2,17 +2,17 @@
 
 #include "tools.h"
 
-#include <QApplication>
-#include <QBrush>
-#include <QColor>
+#include <QtWidgets/QApplication>
+#include <QtGui/QBrush>
+#include <QtGui/QColor>
 #include <QCursor>
-#include <QDebug>
-#include <QFont>
+#include <QtCore/QDebug>
+#include <QtGui/QFont>
 #include <QFontMetrics>
-#include <QMouseEvent>
-#include <QPainter>
-#include <QPen>
-#include <QTime>
+#include <QtGui/QMouseEvent>
+#include <QtGui/QPainter>
+#include <QtGui/QPen>
+#include <QtCore/QTime>
 
 namespace LtsScriptTimeline
 {