feat: p99 and std dev in performance analytics

[eclectic-coding]

Closes #56

Summary

• Extends JobPerformanceStats::Row with p99 and std_dev fields
• percentile(sorted, 99) reuses the existing percentile helper
• std_dev computes population standard deviation via Math.sqrt(Σ(x−μ)²/n); returns 0.0 for single-run jobs
• Both columns appear in the Performance table between p95 and Min, rendered with format_duration
• 2 new specs; full suite at 303 examples, 0 failures, 100% coverage

Test plan

• Visit /jobs/performance with finished jobs — p99 and Std Dev columns appear
• A job class with consistent durations shows a low Std Dev; one with variable durations shows a higher value

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