Add CI performance benchmarks

[wbarnha]

Summary

• Add a Performance Benchmarks GitHub Actions workflow that runs on PRs, pushes to master, weekly schedule, and manual dispatch.
• Compare the current checkout against the two newest non-draft, non-prerelease GitHub release tags.
• Publish stable current-branch benchmark metrics via benchmark-action/github-action-benchmark using customSmallerIsBetter.
• Upload full benchmark artifacts and append a Markdown comparison table to the job summary.

Benchmark coverage

• Message construction
• Message reference count updates
• tp_set_to_map topic-partition grouping

Regression behavior

• Direct release comparison fails the benchmark job when current results exceed the configured slowdown threshold, defaulting to 1.25x.
• Historical github-action-benchmark alerts are posted on pushes to master, but fail-on-alert is disabled to avoid noisy runner variance blocking merges.

Notes

• The benchmark runner uses isolated virtual environments for the current checkout and release tags.
• Release discovery prefers the GitHub Releases API and falls back to local git tags when needed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.98%. Comparing base (706b502) to head (9232f25).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #734   +/-   ##
=======================================
  Coverage   95.98%   95.98%           
=======================================
  Files         103      103           
  Lines       11069    11069           
  Branches     1191     1191           
=======================================
  Hits        10625    10625           
  Misses        350      350           
  Partials       94       94           

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