BUILD_RTI conditional fixes and Qt6 API updates

- cmake/FindRTI.cmake: Add INTERFACE placeholder targets when RTI not found
- cmake/FindRTI.cmake: Force BUILD_RTI=OFF when RTI not installed
- Lts/Common/LtsSession/CMakeLists.txt: Make LtsSessionDataDds conditional on BUILD_RTI
- Lts/Viewer/CMakeLists.txt: Make LtsDataView conditional on BUILD_RTI
- Lts/Plugins/*/CMakeLists.txt: 97 plugin files wrapped in BUILD_RTI conditionals
- cmake/NGLMacros.cmake: Fix NGL_ADD_BIG_RESOURCES macro (removed premature return())
- Multiple Qt6 API fixes: QLinkedList→std::list, QStringRef→QStringView, etc.
- BUILD_RTI_FIXES_SUMMARY.md: Comprehensive documentation of all fixes

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

qt6-after/qt6-migrated-legatus/include/LtsSession/ltssessiondata_export.h

@@ -0,0 +1,21 @@
+/**
+  * Export definitions for LtsSessionData
+  * $Id$
+  * $HeadURL$
+  * $Author$
+  */
+#pragma once
+
+#include <QtCore/qglobal.h>
+
+#ifndef LTSSESSIONDATA_EXPORT
+# ifdef BUILD_STATIC_LIBRARIES
+#  define LTSSESSIONDATA_EXPORT
+# else
+#  if defined(MAKE_LTSESSIONDATA_LIB)
+#   define LTSSESSIONDATA_EXPORT Q_DECL_EXPORT
+#  else
+#   define LTSSESSIONDATA_EXPORT Q_DECL_IMPORT
+#  endif
+# endif
+#endif