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/tests/test_passiveeventcontainer.cpp

@@ -3,8 +3,8 @@
 #include "Lts/Common/LtsTimelineClassic/EventTools.h"
 #include "Lts/Common/LtsTimelineClassic/PassiveEventContainer.h"
 
-#include <QPainter>
-#include <QPixmap>
+#include <QtGui/QPainter>
+#include <QtGui/QPixmap>
 #include <QStyleOptionViewItem>
 #include <gtest/gtest.h>