Implement Async Iterator Helpers

[frostney]

Summary

Implement the Stage 2 TC39 proposal: https://github.com/tc39/proposal-async-iterator-helpers

Implement AsyncIterator.from plus all lazy and terminal helpers in the current draft with correct sequential awaiting and closing.

Why

This proposal fits GocciaScript's modern ECMAScript surface and the 0.13.0 proposal roadmap. It should extend the engine's existing architecture rather than arrive as a userland shim.

Current behavior

Async generators, Symbol.asyncIterator, for-await-of, and Array.fromAsync exist, but the AsyncIterator intrinsic/helper family is absent.

Expected behavior

Implement AsyncIterator.from plus all lazy and terminal helpers in the current draft with correct sequential awaiting and closing.

Scope notes

Reuse async-generator, Promise job, iterator-close, and GC-rooting infrastructure rather than blocking wrappers around sync helpers. Cover async-from-sync, thenables, re-entrancy, abrupt completion, GC, timeouts, docs, and tests. Additive API: no flag.

Related bytecode roadmap: #871 and #875. Update relevant documentation and the Goccia proposal inventory. Use the current proposal specification at implementation time; add focused project tests and matching pinned Test262/proposal coverage without regressions.

Acceptance criteria

• Parser/API surface, descriptors, evaluation order, errors, and protocol behavior match the current proposal.
• Implementation uses the identified shared engine seam without a duplicate compatibility shim.
• Focused positive, negative, abrupt-completion, and edge-case tests cover the proposal.
• Relevant project suites, pinned Test262 cases, formatting, and documentation checks pass.