Cost and latency overhead metrics

[rudrakshkarpe]

Summary

cost_usd and latency_ms exist on AgentTrace but are not scored by any metric. Add metrics that quantify the security-vs-performance tradeoff.

Scope

• LatencyOverhead metric: score based on latency relative to a baseline threshold
• CostEfficiency metric: score based on cost relative to a budget threshold
• Configurable baseline thresholds
• Include in default built_in_metrics() or as opt-in extras

Acceptance Criteria

• Two new metric classes with configurable thresholds
• Integrated into reporting
• Tests covering edge cases (zero latency, missing cost data)