net.websocket integration tests

These tests are intentionally in-memory and synthetic. They exercise websocket.Message processing and callback-shaped control flow with x.async, but they do not start a websocket server or claim end-to-end websocket server coverage.

That limitation is important: in this V snapshot, websocket.Server.close() is not a complete, stable shutdown primitive for a fragile validation test. The tests therefore avoid binding ports and focus on the part x.async can safely compose: message work, cancellation, and error propagation around callbacks.