[Feature] Add async concurrency safety guarantees

[imran-siddique]

Problem

Modern agent frameworks make parallel async tool calls. 50 concurrent policy evaluations could create race conditions or throughput bottlenecks. No thread safety documentation, lock contention profiling, or load testing exists.

Proposed Solution

• Explicit async-safe policy evaluation (asyncio + threading)
• Concurrency benchmarks (10/50/100/500 concurrent evaluations)
• Lock contention profiling results
• Load test results in documentation
• Thread-safety guarantees documented in API reference