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/tests/test_mainwindow.cpp

@@ -0,0 +1,48 @@
+#include <QtTest/QtTest>
+#include <QApplication>
+#include "MainWindow.h"
+
+class TestMainWindowWidget : public QObject
+{
+    Q_OBJECT
+
+private slots:
+    void testMainWindowCreation();
+    void testWindowTitle();
+    void testValidateInputMethod();
+    void testGetScreenInfo();
+};
+
+void TestMainWindowWidget::testMainWindowCreation()
+{
+    MainWindow window;
+    QVERIFY(!window.windowTitle().isEmpty());
+}
+
+void TestMainWindowWidget::testWindowTitle()
+{
+    MainWindow window;
+    QCOMPARE(window.windowTitle(), u"Qt6 Legacy Migration Benchmark"_s);
+}
+
+void TestMainWindowWidget::testValidateInputMethod()
+{
+    MainWindow window;
+    
+    QVERIFY(window.validateInput(u"Test123"_s));
+    QVERIFY(window.validateInput(u"abc456"_s));
+    QVERIFY(!window.validateInput(u"123Test"_s));
+    QVERIFY(!window.validateInput(u""_s));
+}
+
+void TestMainWindowWidget::testGetScreenInfo()
+{
+    MainWindow window;
+    QString info = window.getScreenInfo();
+    QVERIFY(!info.isEmpty());
+    QVERIFY(info.contains(u"Screen:"_s));
+}
+
+QTEST_MAIN(TestMainWindowWidget)
+
+#include "test_mainwindow.moc"