ci: compare PR benchmarks with base

[zeitlinger]

Summary

Make PR benchmark comments answer whether the PR is faster or slower than its base commit, while keeping wall-clock time close to a single benchmark run.

Changes

• Run base and head benchmarks in parallel matrix jobs.
• Add a lightweight summary job that downloads both raw JMH artifacts and generates the PR comment.
• Add a Comparison with base table to the generated README.md / PR comment.
• Mark each benchmark as faster, slower, or within noise using JMH confidence intervals when available.
• Capture and show runner hardware for both base and head jobs, including a note that parallel jobs may run on different hardware.
• Rename the existing per-run relative column to Within run so it is clear that it is not the base comparison.
• Store baseline-results.json in the uploaded benchmark artifact for inspection.

Validation

• Ran mise run lint:fix.
• Ran python3 -m py_compile .mise/tasks/generate_benchmark_summary.py.
• Ran git diff --check.
• Replayed summary generation with the latest perf: optimize classic-only histogram observe path #2252 benchmark artifact and the published benchmark baseline; the output now shows HistogramBenchmark.prometheusClassic as -78.6% / slower.
• Verified no-baseline summary generation still omits the comparison section.

Tradeoff

Parallel base/head jobs are faster wall-clock than measuring both commits sequentially on one runner, but runner hardware can differ. The comment now prints both runner descriptions and calls out that caveat.