Configuration for the Queue / ThreadPool execution modes.
This is threadman's own ThreadPoolOptions, so the full pool surface is available — dynamic scaling (min_workers / max_workers), idle retirement, scale thresholds, a custom name, … Defaults match threadman: max_workers defaults to std::thread::hardware_concurrency() and min_workers to 1, i.e. the pool scales on demand. Set min_workers == max_workers for a fixed-size pool.
max_queue_size is honoured as a blocking back-pressure bound: when set, dispatch() blocks the producer until a slot frees, rather than threadman's native reject-on-overflow (which would drop envelopes). 0 = unbounded.
