fix: class async / async-generator method test262 parity

[proggeramlug]

Summary

Brings the 8 in-class async-method / async-generator-method test262 dirs from 266/516 (51.6%) → 408/516 (79.1%) pass.

Regression-checked against a built-ins+language shard (0/12): +24 net fixed, 0 real regressions (the one shard blip — TypedArray/prototype/forEach/returns-undefined — passes in isolation on both this branch and the merge-base; load-induced flakiness, not from this change). The default-parameter and private-method fixes also lift the standalone language/statements/class/dstr dirs by +23 with zero regressions.

Dirs covered: language/{statements,expressions}/class/{async-method,async-method-static,async-gen-method,async-gen-method-static}.

Root causes fixed

1. No [Symbol.asyncIterator] on async-generator instances. for await over a class async-gen method hung / yielded nothing — GetIterator(obj, async) couldn't resolve the iterator. Install [Symbol.asyncIterator] (returning this) on %AsyncGenerator.prototype%. (The sync %Generator.prototype% deliberately gets none — the sync for-of path doesn't bind implicit-this, so a this-returning thunk would break for (x of gen()).)

2. Async yield* delegated through the sync iterator protocol with no awaits — yield* asyncIterable in an async generator produced nothing or threw "next is not a function". The yield* desugaring is now async-aware: uses GetAsyncIterator and awaits each delegated next() when the enclosing generator is async.

3. GetIterator(obj, async) error semantics. A present-but-non-callable @@asyncIterator, or a callable one returning a non-Object, must throw TypeError — not fall back to the sync iterator. Rewrote js_get_async_iterator to the spec ordering.

4. Missing trailing args padded with NaN instead of undefined in dynamic vtable / static-method dispatch. Default params lower to param === undefined ? <default> : param, so a hole padded with NaN never triggered the default (e.g. a detached C.prototype.method called with fewer args).

5. Private methods never applied parameter defaults (direct or via a this.#m value): lower_private_method built param.default but never prepended the if (param === undefined) param = default prologue public methods emit. Fixes all private method kinds.

6. Latent: small typed arrays are raw-alloc'd with no object GcHeader, so Object.isExtensible read garbage _reserved and non-deterministically reported false. Integer-indexed exotic objects are extensible by default; short-circuit TAs/buffers.

Files

• perry-transform/src/generator/linearize.rs, lower.rs — async-aware yield* delegation
• perry-runtime/src/object/global_this.rs — %AsyncGenerator.prototype%[Symbol.asyncIterator]
• perry-runtime/src/array/iterator.rs — spec GetIterator(obj, async)
• perry-runtime/src/object/class_registry.rs — undefined (not NaN) arg padding
• perry-hir/src/lower_decl/private_members.rs — private-method default prologue
• perry-runtime/src/object/object_ops_frozen.rs — TA/buffer isExtensible

Notes for the maintainer

• Branch is based on cf56a6faf; current main has #4766 (dstr + dflt-params). Fix useEffect + setState panics with RefCell already borrowed on macOS ARM64 #4 (call_vtable_method NaN→undefined) is still needed on current main and shouldn't conflict.
• Remaining failures here are mostly fine-grained async yield* execution-order tests (custom getter/thenable ordering) that conflict with Perry's synchronous-await-drain model, plus static-method this binding (a separate, broad pre-existing gap: static methods see this === undefined).
• Kept scoped to the async-method state machine + shared dispatch; expect the usual rebase at merge.