[Feature]: Add benchmark trend tracking across CI runs

[MisterVVP]

Proposal

Benchmark signal

The current benchmark job catches threshold regressions, but it does not show trend history across commits.

Task

Persist benchmark summaries for trend analysis.

Scope

• Upload benchmark-results.json and benchmark-summary.md as artifacts already exists; extend this with a compact trend JSON.
• Optionally publish a GitHub Pages benchmark history.
• Track mean/median for key benchmarks over time.
• Add a script to compare current run against previous successful main run.

Success criteria

• PRs can show “changed by +X% / -Y%” for key benchmarks.
• Avoid overreacting to one noisy run.
• Keep hard thresholds as guardrails, but use trend data for roadmap decisions.