0508 — The concurrency package goes internal; consumers build queue and pool from ConsumerConfig

Context. concurrency.Queue, PoolLimiter, PressureQueue, and WorkerPoolLimiter were exported, and consumer constructors took a queue and a pool limiter as params — forcing users to assemble internal machinery and leaking consumer.Buffered through NewPressureQueue[consumer.Buffered].

Decision. The whole concurrency package is hidden. Consumers build their queue and pool internally from ConsumerConfig; the constructors drop the two params, and the consumer.Buffered leak goes with them.

Consequences. Users tune concurrency through config fields, not by constructing internals; the package’s types stop being API the library must keep stable.