thread io: Io through Server.init → WorkerState → ConnManager → Conn, replacing hardcoded std.Options.debug_io. add Server.runIo() which uses Io.net.Server.accept + io.concurrent per connection — under Evented this becomes fiber-based (no OS thread per connection), under Threaded it behaves like the old blocking path. existing listen()/listenInNewThread() preserved for backwards compat. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>