refactor(core): simplify listen() API by internalizing Io setup

[codev911]

Summary

• Move std.Io.Threaded initialization from every call site into listener.listenAndServe(), eliminating the need for users to manually create and pass an
  io instance
• Simplify App.listen() signature from listen(self, io, address) to listen(self, address)
• Update all 9 examples + README to reflect the cleaner API

Changed files

• src/core/listener.zig — listenAndServe() now creates its own std.Io.Threaded internally
• src/core/app.zig — listen() no longer accepts io parameter
• examples/*.zig (8 files) — remove manual threaded/io boilerplate
• README.md — update quickstart snippet