Add std_latency to LatencyBenchmark metrics. New _render_stats_table
in bench_cmd.py detects stats-pattern keys (mean_X, p50_X, min_X,
max_X, std_X, p95_X) and renders Avg/Median/Min/Max/Std/P95 columns.
Falls back to simple key-value for non-stats metrics.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>