perry-stdlib: ReadableStream / Web Streams API not implemented #320

What happened

After Effect compiles past codegen (post #309 / #310), the link step fails with:

Undefined symbols for architecture arm64:
  "_js_readable_stream_new",
      referenced from: <Effect's Stream module>
  "_js_readable_stream_controller_close",
  "_js_readable_stream_controller_enqueue",
  "_js_readable_stream_controller_error",

These are 4 FFI symbols that perry-codegen emits calls to but perry-stdlib does not provide. They correspond to the WHATWG ReadableStream constructor + controller methods.

What you expected

Either:

1. Implement ReadableStream in perry-stdlib (alongside fetch, URL, etc. from the WHATWG family). Effect's Stream module — and any Node 18+ code touching fetch().body — needs this.
2. Detect at codegen time that no implementation exists and either emit a clear "ReadableStream not yet supported" error at the call site, or fall through to a runtime stub that throws on use rather than producing a link error.

Likely fix site

• Codegen emits these symbols in crates/perry-codegen/src/lower_call.rs (search for js_readable_stream_) and / or wherever ReadableStream resolves as a built-in.
• Implementation belongs in crates/perry-stdlib/src/streams.rs (new file), modeled after fetch.rs / buffer.rs for the FFI shape.
• Real ReadableStream implementation is non-trivial (backpressure, queuing strategies, async iteration). A minimum-viable implementation matching Node 18's node:stream/web surface is the right scope.

Minimal reproduction

const stream = new ReadableStream({
  start(controller) {
    controller.enqueue("hello");
    controller.close();
  }
});
console.log(stream);

perry compile repro.ts -o /tmp/out
# fails at link with: Undefined symbols: _js_readable_stream_new, ...

Environment

• Perry version: 0.5.405
• Host OS: macOS 26.4 (arm64)
• Target: native
• Effect version: 3.21.2 (where this surfaced)

Adjacent gaps to consider in the same fix

While doing the WHATWG-streams audit, also check:

• WritableStream (likely emitted but unimplemented too)
• TransformStream
• ReadableStreamDefaultReader / ReadableStreamDefaultController
• pipeTo / pipeThrough
• Response.body (currently unwired — see response.blob() drops body bytes — same shape as #227, needs Blob instance methods #234 follow-ups)

Discovered as part of the #309 long tail.