Implement Iterator.prototype.join

[frostney]

Summary

Implement the Stage 3 TC39 proposal: https://github.com/tc39/proposal-iterator-join

Add proposal-correct default/custom separators, string coercion, nullish element handling, single consumption, and abrupt 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

The synchronous Iterator surface has no join terminal operation.

Expected behavior

Add proposal-correct default/custom separators, string coercion, nullish element handling, single consumption, and abrupt closing.

Scope notes

Implement alongside existing terminal helpers without materializing the iterator; add descriptor/coercion/closing tests, docs, and Test262 coverage. Additive API: no feature 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.