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/Services/RoutingServiceNG/data_writer_task.h

@@ -16,6 +16,8 @@
 
 #if BUILD_RTI
 #include <DDS/Routing/routing_plugin.h>
+#else
+#include "stubs/routing_plugin_stub.h"
 #endif
 
 namespace RoutingService
@@ -35,10 +37,10 @@ public:
   void writeSamples();
 #else
   // Stub methods when BUILD_RTI is disabled
-  void enqueue(void* /*swi*/) override {}
+  void enqueue(SampleWriterInterface* /*swi*/) override {}
   void shutdown() {}
   bool waitForData() { return false; }
-  void *getNextSampleWriter() { return nullptr; }
+  SampleWriterInterface* getNextSampleWriter() { return nullptr; }
   void writeSamples() {}
 #endif