Build a small `pick_concurrency(work_type)` that returns one of `"thread"`, `"process"`, or `"asyncio"`: - `"io_bound"` → `"asyncio"` (modern Python's default for I/O) - `"cpu_bound"` → `"process"` (GIL means threads can't parallelise pure-Python compute) - `"network_io"` → `"asyncio"` (same family as io_bound — high concurrency at low overhead) - `"file_io_legacy"` → `"thread"` (sync stdlib like `open()` doesn't release the GIL the way socket I/O does; ThreadPool is the pragmatic fix) - anything else → `"thread"` (safe default: doesn't require event-loop discipline)