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/Proxies/TrainingAreaBase/training_area_telegrams.h

@@ -2,7 +2,7 @@
 
 #pragma once
 
-#include <QString>
+#include <QtCore/QString>
 #include <ctime>
 #include <string>
 #include <vector>
@@ -10,8 +10,9 @@
 #include <bitset>
 #include <unordered_map>
 #include <unordered_set>
+#include <ostream>
 
-#include <QJsonObject>
+#include <QtCore/QJsonObject>
 
 namespace TrainingAreaBase
 {
@@ -372,11 +373,8 @@ enum class UAVState
 
 struct Header
 {
-  int m_equipmentNumber = 0;
-  qint64 m_timeStamp = 0;
-
-  int equipmentNumber() const { return m_equipmentNumber; }
-  qint64 timeStamp() const { return m_timeStamp; }
+  int equipmentNumber = 0;
+  qint64 timeStamp = 0;
 
   static Header fromJson(const QJsonObject &obj);
   QJsonObject toJson() const;
@@ -2301,7 +2299,6 @@ struct BatteryAlarm : TrainingAreaTelegramTmpl<BatteryAlarm>
   qint64 m_alarmTime;
 
   Header header() const { return m_header; }
-  TelegramId id() const { return static_cast<TelegramId>(m_id); }
   QString description() const { return m_description; }
   bool acknowledge() const { return m_acknowledge; }
   bool condition() const { return m_condition; }