Bounded concurrency via semaphore+WaitGroup, atomic counters, stop-on-first- error without cancelling in-flight requests (cancel would hide the leaks). Exposes MaxIdleConnsPerHost so pool thrash under concurrency is observable. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01GbnJj9ZqEx9nUvzkjdwfMq