`Report.get_summary()` iterates over `results` five separate times

[deyna256]

get_summary() makes five independent passes over self.results:

1. sum(1 for r in self.results if r.passed) — passed count
2. [r.metrics.latency_ms for r in self.results if r.metrics] — latencies list
3. sum(r.metrics.cost_usd or 0 for r in self.results if r.metrics) — total cost
4. sum(r.metrics.total_tokens or 0 for r in self.results if r.metrics) — total tokens
   5–6. _count_execution_errors() and _count_validation_failures() — two more full iterations

All six values can be computed in a single pass. As the result set grows, the current implementation performs O(6n) work where O(n) is sufficient.