feat(stdlib,lowering): implement WHATWG IterableIterator and generic Array.from/for..of iterator support

[phamtanminhtien]

Summary

This PR establishes full WHATWG and ECMAScript spec compliance for iterators and collections:

1. WHATWG IterableIterator Implementation:

   • FormDataIterator<T>, HeadersIterator<T>, and URLSearchParamsIterator<T> implement ECMAScript IterableIterator<T> protocol (.next(): { value: T | undefined; done: boolean } and [Symbol.iterator]()).
   • FormData, Headers, and URLSearchParams methods (keys(), values(), entries(), and [Symbol.iterator]()) return typed iterator instances conforming to WHATWG standards.
   • Updated globals.d.ts interface definitions extending IterableIterator<T>.

2. Generic for..of & Array.from Iterator Support:

   • Refactored lowerForOf in statements_control.go to dynamically resolve next and Symbol.iterator methods via findMethodInHierarchy.
   • Extended Array.from in builtins_array.go to consume arbitrary iterables/iterators via .next() loop protocol, Set (__set.values), and Map (__map.entries).
   • Dynamically resolves done and value fields by name in shape descriptors instead of hardcoding indices.

3. Generic Monomorphization & Type System Hardening:

   • Added nested bracket ([]), brace ({}), and paren (()) depth awareness to splitTypeArguments in generics_subst.go.
   • Added object literal type ({ ... }) parsing and substitution to substituteType.
   • Used splitTopLevelUnion instead of blind delimiter splitting for union types.
   • Preserved structured shapes and resolved type aliases during monomorphization.

4. Verification:

   • Full compiler corpus tests passed (api/formdata.ts, api/headers.ts, api/urlsearchparams.ts, api/fetch.ts, api/request.ts, api/response.ts, etc.).
   • 100% full Node.js v22 parity via cmd/parity.
   • ASAN & UBSAN clean (zero leaks, zero runtime errors).
   • Full repository test suites passed.