node:util: expose transferable AbortSignal helpers #3419

Summary

util.transferableAbortController() and util.transferableAbortSignal(signal) should be callable Node helpers for creating/marking AbortSignals that can be transferred through structured clone and worker-message paths. Perry currently rejects the helper at compile time as an unimplemented node:util API.

Node Behavior

Minimal repro:

import * as util from "node:util";

console.log(typeof util.transferableAbortController);
console.log(typeof util.transferableAbortSignal);

const controller = util.transferableAbortController();
console.log(typeof controller.abort, controller.signal.aborted);
controller.abort("done");
console.log(controller.signal.aborted, controller.signal.reason);

const ac = new AbortController();
const signal = util.transferableAbortSignal(ac.signal);
console.log(typeof signal, signal === ac.signal, signal.aborted);
ac.abort("later");
console.log(signal.aborted, signal.reason);

Node v25.9.0 prints function-shaped helpers, a controller with a live signal, and a signal that reflects later aborts:

transferableAbortController typeof: function
transferableAbortSignal typeof: function
controller abort typeof: function
controller signal aborted: false
controller signal after abort: true done
signal object: object true false
signal after source abort: true later

Perry Behavior

A focused parity probe in test-files/tmp_util_transferable_abort.ts fails to compile:

Error: `util.transferableAbortController` is not implemented in Perry — see `perry --print-api-manifest` for the supported surface, or set `PERRY_ALLOW_UNIMPLEMENTED=1` to ignore. (#463)

Report: test-parity/reports/parity_report_20260530_103958.json.

This is listed in the broad util helper tracker #2514, but it deserves a focused leaf because it is the AbortSignal/structured-clone cancellation subset rather than the unrelated debug/system-error/string-format helpers in that bucket.

Suggested PR Cut

Batch this with related issues in:
node:util: cancellation and structured-clone helper parity

Good batch candidates:

• node:util: implement remaining top-level helper APIs #2514
• globals: complete AbortSignal static helpers and abort lifecycle #2582

Do not batch with:

• unrelated node:util formatting/debug/system-error helpers
• broad worker_threads implementation work
• general structuredClone iterator/object-shape fixes

Acceptance

• parity/regression test proves util.transferableAbortController and util.transferableAbortSignal are function-valued exports
• transferableAbortController() returns an AbortController-like object whose signal aborts with the supplied reason
• transferableAbortSignal(signal) returns/marks a signal that preserves abort propagation and reason behavior like Node
• implementation matches Node observable behavior
• related known-failure/docs/manifest entries updated if touched