feat(js): add lifecycle-managed js.io() to the JS DSL

[nazarhussain]

Motivation

We already expose lifecycle-scoped DSL context helpers like js.env() and js.allocator(). This change adds the analogous I/O surface so DSL code can use std.Io facilities without requiring module authors to bootstrap or own a separate runtime.

Summary

• adds a public js.io() accessor to the JS DSL
• backs js.io() with a shared std.Io.Threaded instance retained for the lifetime of active addon environments
• wires retain/release into js.exportModule(...) so consumers do not need any manual setup

API Notes

• the public API is js.io()
• retain() / release() remain internal implementation details
• module consumers should not call any manual retain API
• js.io() is only valid during the normal addon lifecycle window and will panic if used before registration or after final cleanup