Add async concurrency benchmark and validation script

Validates 7 properties of the async client against a live environment:
1. Non-blocking reads — canary confirms event loop stays free during GETs
2. Read throughput — concurrent reads via gather() beat sequential
3. Write concurrency — concurrent POSTs beat sequential (POST path)
4. Pagination non-blocking — async generators yield between page fetches
5. Mixed fan-out — different op types run simultaneously without serialization
6. Error resilience — one failure in gather() does not kill other calls
7. Real-world fan-out — metadata for multiple tables fetched in parallel

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Abel Milash