Migrate from zio to std.Io

[lalinsky]

Summary

• Replace zio dependency with Zig 0.16.0's built-in std.Io
• Update all APIs to pass io: std.Io parameter
• Update CI to use Zig 0.16.0
• Update test_runner for std.Io compatibility

Changes

• zio.Duration → std.Io.Duration
• zio.Timeout → std.Io.Timeout
• zio.sleep() → io.sleep(duration, .awake)
• zio.net.tcpConnectToHost() → std.Io.net.HostName.init().connect()
• zio.Mutex → std.Io.Mutex with lock(io), unlock(io)
• Main entry point: pub fn main() !void → pub fn main(init: std.process.Init) !void