ci: upload callgrind diff summary

[hardfist]

Summary

• Generate a Callgrind diff report for the largest Valgrind Callgrind data file produced by the Rust benchmark job.
• Download the latest matching main-branch Valgrind artifact and compare it against the current run using callgrind_annotate plus callgrind_differ.
• Append the report to the GitHub Actions job summary and keep the report files in the uploaded Valgrind artifact.

Validation

• git diff --check -- .github/workflows/bench-rust.yml .github/workflows/ci.yml
• bash -n over the updated workflow shell block
• cargo install --git https://github.com/Ethiraric/callgrind_differ --locked --bin callgrind_differ --root <tmp>

[github-actions]

📦 Binary Size-limit

Comparing c47b20f to fix(cli): remove process title startup overhead (#14061) by Jiahan Chen

🙈 Size remains the same at 61.98MB

[github-actions]

Rsdoctor Bundle Diff Analysis

Found 6 projects in monorepo, 0 projects with changes.

📊 Quick Summary

Project	Total Size	Change
popular-libs	1.7 MB	0
react-10k	5.7 MB	0
react-1k	826.3 KB	0
react-5k	2.7 MB	0
rome	1.6 MB	0
ui-components	4.8 MB	0

Generated by Rsdoctor GitHub Action

[codspeed-hq]

Merging this PR will not alter performance

✅ 34 untouched benchmarks
⏩ 25 skipped benchmarks¹

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_{Comparing codex/callgrind-differ-summary (c47b20f) with main (361ecde)}

Footnotes

1. 25 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩