feat: Add LCOV coverage support for Jenkins CI

- Add ENABLE_COVERAGE CMake option with gcov flags
- Create collect-coverage.sh script for LCOV workflow
- Add Qt6 test files with QTest framework
- Update Jenkinsfile with Coverage stage (main/master/coverage.*)
- Update CI Docker image with lcov and genhtml packages
- Add .gitignore entries for coverage artifacts
- Add COVERAGE.md documentation

Coverage workflow:
1. Configure with -DENABLE_COVERAGE=ON
2. Build and run tests to generate .gcda/.gcno files
3. Collect coverage with lcov
4. Filter out Qt/system libraries
5. Generate HTML report and lcov.info for Jenkins

Jenkins publishes coverage/lcov.info using Coverage Plugin.
Coverage stage only runs on protected branches.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Entire-Checkpoint: 7783c7698334

qt6-after/qt6-migrated-benchmark/main.cpp

@@ -1,19 +1,11 @@
+#include <QApplication>
 #include "MainWindow.h"
 
-#include <QApplication>
-#include <QScreen>
-
-#if __has_include(<print>)
-#include <print>
-#endif
-
-[[nodiscard]] auto main(int argc, char* argv[]) -> int
+int main(int argc, char *argv[])
 {
-    QApplication app{argc, argv};
+    QApplication a(argc, argv);
+    MainWindow w;
+    w.show();
 
-    // Qt6 uses UTF-8 by default; no codec setup needed
-    MainWindow window;
-    window.show();
-
-    return app.exec();
+    return a.exec();
 }