docs: add manager concurrency benchmarks

[LYJW131]

What changed

• add a standalone CONCURRENCY_BENCHMARK.md
• document the isolated concurrency methodology used on z16 and rog
• include host/Docker specifications and the complete pool 4/6/8/10/12/16 results
• include reverse-order and multi-ClientConn cross-checks
• record the production recommendation to keep 10 streams per wrapper

Why

The concurrency limit was previously justified only through discussion and local test artifacts. This report keeps the measured evidence and operating recommendation next to the manager source.

Validation

• go test ./...
• Markdown table geometry and heading checks
• numerical assertions against both benchmark summaries
• git diff --check
• verified the report contains no account, IP, credential path, token, or secret