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.

.gitignore

@@ -3,3 +3,8 @@
 *.o
 *.a
 chunk_server_data_*
+# Coverage reports
+coverage_report/
+*.gcov
+*.gcda
+*.gcno