Bench

Run performance benchmarks to get a verdict on whether a kernel change actually helped. Use --ab-compare for sub-1x deltas (drift cancels in same container), --variance-check for noise floor, and the subset modes (--first, --smoke; --extremes is modal-backend only) for compile-correctness probes only — those are NOT performance verdicts. The only authoritative source for "is this change real". Benchmark specifics (config.toml schema, status enum, scoring formula, baseline rule, fresh-inputs contract) live in the `benchmark` skill.