Add HTML coverage reports with branch-level detail

Enhance the branch coverage measurement tool with HTML report generation
that shows exactly which branches were taken during simulation execution.

Changes:
- Add generate_coverage_html.sh script to create interactive HTML reports
- Update measure_coverage.sh with --html flag to generate reports
- Add *.gcov, *.gcda, *.gcno to .gitignore
- HTML reports show:
  - Overall coverage summary with visual progress bars
  - Per-file coverage breakdown with clickable links
  - Source code view with color-coded branch coverage
  - Green highlight: branches taken
  - Red highlight: branches not taken
  - Branch execution counts and percentages

Usage:
  ./measure_coverage.sh [duration] --html

The HTML report is generated in coverage_report/index.html and can be
viewed in any web browser. Each source file links to a detailed view
showing which specific branches were executed.

measure_coverage.sh

@@ -92,15 +92,20 @@ fi
 echo -e "${BLUE}========================================${NC}"
 echo
 
-# Optionally show detailed coverage for specific files
-if [ "$2" == "--detailed" ]; then
-    echo -e "${YELLOW}Detailed coverage reports available in *.gcov files${NC}"
-    echo "Use 'less <filename>.gcov' to view detailed line and branch coverage"
+# Generate HTML report if requested
+if [ "$2" == "--html" ]; then
+    echo -e "${YELLOW}Generating HTML coverage report...${NC}"
+    ./generate_coverage_html.sh
+    echo -e "${GREEN}HTML report generated in coverage_report/index.html${NC}"
+    echo "Open with: firefox coverage_report/index.html (or your preferred browser)"
+    echo
 fi
 
-# Clean up gcov files
-echo -e "${YELLOW}Cleaning up coverage files...${NC}"
-rm -f *.gcov
+# Clean up gcov files unless HTML was requested
+if [ "$2" != "--html" ] && [ "$2" != "--detailed" ]; then
+    echo -e "${YELLOW}Cleaning up coverage files...${NC}"
+    rm -f *.gcov
+fi
 
 echo
 echo -e "${GREEN}Coverage measurement complete!${NC}"