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

Jenkinsfile

@@ -15,6 +15,7 @@ pipeline {
   environment {
     PROJECT_DIR = "qt6-after/qt6-migrated-benchmark"
     BUILD_DIR = "build"
+    COVERAGE_BUILD_DIR = "build-coverage"
     CHANGE_TARGET = "${env.CHANGE_TARGET ?: 'origin/main'}"
     QT_QPA_PLATFORM = "offscreen"
     LANG = "C.UTF-8"
@@ -76,6 +77,26 @@ pipeline {
         sh 'QT_QPA_PLATFORM=offscreen LANG=C.UTF-8 LC_ALL=C.UTF-8 ctest --test-dir ${BUILD_DIR} --output-on-failure'
       }
     }
+
+    stage('Coverage') {
+      when {
+        expression {
+          return env.BRANCH_NAME ==~ /main|master|coverage.*/
+        }
+      }
+      steps {
+        echo "Running coverage collection..."
+        sh '''
+          ${PROJECT_DIR}/scripts/collect-coverage.sh ${WORKSPACE}/${COVERAGE_BUILD_DIR}
+        '''
+      }
+      post {
+        always {
+          publishCoverage adapters: [lcovAdapter(mergeToOneReport: true, reportFile: 'coverage/lcov.info')],
+                        sourceFileResolver: sourceFiles('STORE_LAST_BUILD')
+        }
+      }
+    }
   }
 
   post {