fix(report): render charts for single-role runs and missing validator metrics

[meyer9]

Summary

The run-comparison charts page assumed every run has both sequencer and validator metrics. For sequencer-only runs (no metrics-validator.json) the charts page broke in two ways. This PR makes it degrade gracefully so you can always compare runs — including grouping by Transaction Payload when only one role is present.

Problems fixed

1. Missing-validator blanked the whole page. ChartSelector fabricated both a sequencer and a validator series for every run. Fetching the non-existent validator metrics 404'd, and that rejected the entire Promise.all batch in useMultipleDataSeries, so no charts rendered.

   • Now: a role series is only emitted when that role's metrics exist (result.sequencerMetrics / result.validatorMetrics), and an individual failed fetch degrades to an empty series instead of failing the batch. ChartGrid already skips empty series.

2. Group-by broke with a single role. The default group-by is role, but a dimension with only one value is dropped from the available variables. The default then pointed at an unavailable dimension, and the role memo threw on variables.role[0] (undefined), breaking every non-role group-by.

   • Now: useBenchmarkFilters falls back to the first available variable (e.g. TransactionPayload) when the requested group-by isn't available, and guards the role lookup with a data-derived fallback. So comparing by Transaction Payload works with a single role.

Changed files

• report/src/components/ChartSelector.tsx — only emit role variants that have metrics; bind UI to the effective group-by.
• report/src/hooks/useBenchmarkFilters.ts — effectiveByMetric fallback + safe role memo.
• report/src/utils/useDataSeries.ts — per-series graceful failure instead of batch rejection.
• report/src/pages/RunComparison.tsx — keep completed runs chartable regardless of the success flag.

Testing

• tsc --noEmit: no new errors in the changed files.
• eslint: clean on the changed files.
• Verified against a real sequencer-only 6-run comparison: charts render and "Show Line Per → Transaction Payload" shows one line per run.

Opened as draft for review.

[cb-heimdall]

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