Fetch-only async I/O

Date: 2026-04-24 Area: runtime

Fetch-only asynchronous I/O. fetch() now returns a pending Promise before network completion and runs blocking HTTP work on fetch-specific background workers. Completions are settled back on the owning runtime thread, then normal Promise reactions drain through the existing microtask queue; the microtask queue remains a Promise-reaction queue, not an I/O queue. await fetch(...) deliberately keeps the current synchronous await model by pumping fetch completions while it waits; a general event loop and true async continuations remain future work. built-ins.md § fetch.