feat: add sequencer-only benchmark role

[niran]

Summary

• Add benchmark-level roles config while preserving the default sequencer+validator behavior.
• Support roles: [sequencer] for benchmarks that only need sequencer/block-building or snapshot startup coverage.
• Normalize config roles into an internal execution mode where the sequencer phase always runs and validator replay is optional.
• Reject validator-only configs because validator benchmarks consume payloads produced by the sequencer phase.
• Reject validator-scoped thresholds for sequencer-only benchmarks.
• Make validator metrics optional in run metadata.
• Document the sequencer-only snapshot/load-test use case.

Tests

• go test ./runner/benchmark
• go test ./runner/...

type=routine
risk=low
impact=sev5

[cb-heimdall]

✅ Heimdall Review Status

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Reviews	✅ 1/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1

[wlawt]

nit: wonder if we can just call the inner content of this fn instead?