fix: clarify benchmark regression report verdicts

[zeitlinger]

Summary

• Add an explicit base-vs-head regression verdict using comparable JMH metadata, confidence-interval overlap, and a practical 5% threshold.
• Remove cross-method Nx slower rankings from the PR-head tables.
• List head-only benchmarks separately as descriptive results with no regression verdict.

Motivation

The #2329 benchmark report described HistogramBenchmark.openTelemetryExponential as “20x slower” only because it was ranked against a different benchmark method in the same run. Its actual base/head delta was -5.7% with overlapping confidence intervals (within noise). This change makes that distinction explicit and prevents the within-run ranking from being mistaken for a regression.

Tests

• python3 .mise/tasks/test_generate-benchmark-summary.py
• python3 .mise/tasks/test_update-benchmarks.py
• ruff check and ruff format --check on the changed Python files
• mise run lint (full lint is blocked only by the pre-existing README Slack link returning HTTP 403; the pre-push scoped lint passed)

Related: #2329.