Add direct snprintf vs ctprintf benchmark comparison page

[Copilot]

The benchmark dashboard on gh-pages only plots each benchmark's history on its own chart. It doesn't put snprintf and ctprintf next to each other. This PR adds a side-by-side comparison generated from the same benchmark run.

• scripts/benchmark-comparison.py (new)

  • Reads the Google Benchmark JSON and pairs benchmark_snprintf_<case> with benchmark_ctprintf_<case>.
  • For each pair it reports CPU time for both and the ratio (Nx faster or Nx slower for ctprintf).
  • Outputs a static comparison.html with paired bars, plus a markdown table.
  • If the run has repetitions, it uses the mean aggregate and ignores the other aggregates.
  • A new case shows up automatically if it follows the same benchmark_{snprintf,ctprintf}_<case> naming. Unpaired benchmarks are skipped.

• .github/workflows/benchmark.yml

  • Appends the table to $GITHUB_STEP_SUMMARY.
  • Commits comparison.html to the root of gh-pages, next to the existing history dashboard. The page links back to that dashboard.
  • The commit and push step runs after github-action-benchmark, which also pushes to gh-pages. The existing concurrency group serializes runs, so I don't expect the two pushes to race.

• README.md: links to the comparison page.

The benchmark sources and the existing dashboard are unchanged.

I only ran the script locally against a small hand-written results file. The workflow steps and the gh-pages publish haven't been exercised in CI yet.