chore(bench): add GFQL benchmarks and CI job

[lmeyerov]

⚠️ Restack in progress. Do not merge this PR. Replacement PR: #918.

Summary

• add GFQL benchmark scripts and docs under benchmarks/
• add CI job to run a small GFQL benchmark subset on GFQL/benchmark changes
• upload gfql-bench.md as a GitHub Actions artifact

Stack

• Base: master
• Parallel: feat(otel): tracing helpers and header propagation #910 (otel)
• Stack note: perf(hop): traversal optimizations and fast path #911/feat(gfql): add same-path WHERE executor #912 are merged into stack branches (not master yet)
• Next: Docs: GFQL WHERE reference #914 (docs, based on this PR)

Testing

• not run locally (CI job added to cover benchmarks)

[lmeyerov]

Superseded by #918 (restack).