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/Lts/Viewer/SituationMap/SymbologyXML/Symbole/AddOnSymbole/vbgHit.xml

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE symboltable SYSTEM "symbole.dtd">
+<symbol version="1.0" id="0x00001000" dimen="undefined" affil="neutral" bcdab="undefined" command_level="undefined" mob="undefined">
+  <name>vbgHit</name>
+  <graphic scale="1">
+    <line transform="false" rotate="0" linewidth="0.15" linestyle="solid">
+      <point x="-1.6" y="0"/>
+      <point x="1.6" y="0"/>
+    </line>
+  </graphic>
+</symbol>