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_select_handler.cpp

@@ -4,13 +4,13 @@
 #include "track_content_widget.h"
 #include "track_entry.h"
 
-#include <QCoreApplication>
-#include <QDebug>
-#include <QGuiApplication>
-#include <QInputDialog>
-#include <QKeyEvent>
-#include <QMenu>
-#include <QMouseEvent>
+#include <QtCore/QCoreApplication>
+#include <QtCore/QDebug>
+#include <QtGui/QGuiApplication>
+#include <QtWidgets/QInputDialog>
+#include <QtGui/QKeyEvent>
+#include <QtWidgets/QMenu>
+#include <QtGui/QMouseEvent>
 
 namespace LtsScriptTimeline
 {