feat(runner): saturated-window GPS metric + always-emit sequencer results

[meyer9]

Summary

Two related changes that make sequencer benchmark results trustworthy and always reported. Motivated by running the harness against very large state (hundreds of millions of accounts), where the post-run block-replay phase reliably times out and the current code discards the whole run's metrics as a result.

Always-emit sequencer results

• A load-test payload worker that exits non-zero after producing a valid result is now treated as a workload failure, not a fatal error: the run keeps its collected sequencer metrics and records a FailureReason instead of throwing everything away.
• service.runTest salvages GetResult() when the run errors (e.g. the post-run newPayload phase hits a context deadline on huge state), so the collected per-block metrics are still emitted with success=false.
• RunResult gains FailureReason; GetResult derives Success from the workload outcome instead of hardcoding true.

Saturated-window gas/sec

• gasPerSecond is now averaged only over saturated blocks (gas ≥ 0.5× the median block gas), so warm-up/drain blocks no longer dilute the headline number. It no longer keys off the nominal gas limit (real blocks never reach it, which previously produced a zero).
• Adds a regime label (cadence-limited / overloaded / unsaturated) derived from mean getPayload latency vs. block time, plus a SaturatedBlockCount, so a reader can tell whether the builder actually kept up.
• Records a per-block duration/block_building metric used by the aggregate.

Notes for review

• Base is testing/local-zfs-runner (this change builds on that branch; retarget to main once it lands).
• Companion report-side fix (renders these single-role runs) is in fix(report): render charts for single-role runs and missing validator metrics #204.
• go build ./... and go vet ./... clean; my files are gofmt-clean (two unrelated pre-existing unformatted files on the branch are left untouched).

Draft for review.