benchmark workflow: render a pretty summary in the GitHub Actions job summary

[matthewdevenny]

Problem

The benchmark workflow (.github/workflows/benchmark.yml, added in #47) runs the serialized-size report and the BenchmarkDotNet timings, but results only land in the raw job log:

- name: Serialized size report
  run: dotnet run -c Release --project util/Benchmarks -f net10.0 -- --sizes
- name: Benchmarks (net10.0)
  run: dotnet run -c Release --project util/Benchmarks -f net10.0 -- --filter "${FILTER:-*}"
- name: Benchmarks (net8.0)
  run: dotnet run -c Release --project util/Benchmarks -f net8.0 -- --filter "${FILTER:-*}"

To read the numbers you have to open the log and scroll. Since this is a manual, on-demand workflow whose entire purpose is to eyeball results, they should render as formatted markdown tables on the run's Summary page ($GITHUB_STEP_SUMMARY).

Proposal

Write the results to $GITHUB_STEP_SUMMARY as GitHub-flavored markdown:

• BenchmarkDotNet timings — BenchmarkDotNet already emits a GitHub-markdown artifact per run at BenchmarkDotNet.Artifacts/results/*-report-github.md (or force it with --exporters github). After each run step, append those files to the summary, e.g.:

  {
    echo "### Benchmarks (net10.0)"
    cat BenchmarkDotNet.Artifacts/results/*-report-github.md
  } >> "$GITHUB_STEP_SUMMARY"

  Tag each net10.0 / net8.0 section with a heading so both TFMs are distinguishable, and clear/segregate the artifacts dir between the two runs so they don't clobber or double-append.
• Serialized-size report — SizeReport.Print() currently writes a fixed-width plaintext table to the console. Either (a) add a markdown-table output mode (e.g. --sizes --md) that the workflow redirects into the summary, or (b) wrap the existing output in a fenced code block when appending. Option (a) is nicer since the size/ratio table is the headline result.

Acceptance criteria

• After a benchmark run, the GitHub Actions Summary page shows the size-comparison table and the net10.0 + net8.0 BenchmarkDotNet tables as rendered markdown, without opening the raw log.
• net8.0 and net10.0 results are clearly labeled and not intermixed.
• Raw-log output is preserved (the summary is additive, not a replacement).

Notes

No new dependencies needed — BenchmarkDotNet ships the GitHub markdown exporter. Keep the FILTER env-var indirection (shell-injection guard) intact when touching the run steps.