fix(runtime): class subclass + async-method + definition test262 tail

[proggeramlug]

Summary

Fixes the class test262 tail across subclass / async-method / definition / syntax. Target suite (15 dirs) 662 → 685 pass (+23), parity 84.8% → 87.7%, zero regressions (verified via --shard 0/12 over built-ins language + isolated re-runs of every concurrent-shard flaky diff).

Root causes & fixes

1. Methods/getters/setters/bound functions had a prototype own property. ordinary_function_prototype_value_for_read synthesized .prototype for any non-arrow callable incl. bound-method values (C.prototype.m, c.m, fn.bind()). Added closure_is_bound_method and suppress it. (definition/methods, numeric-property-names)
2. Derived super(message) into a built-in Error set message enumerable. Spec uses DefinePropertyOrThrow {enumerable:false}. Added js_object_set_field_by_name_nonenum, routed all 3 Error-init codegen sites through it. (NativeError/*-message, Error/message-property-assignment)
3. Static gen/async method .length over-counted. New CLASS_STATIC_METHOD_BIND_LENGTHS table (distinct from instance table) + codegen registration with default-aware spec length. (*-method-static dflt-params-trailing-comma)
4. class X extends <non-constructor> didn't throw TypeError. New extends_target_must_throw (null OK; primitive/arrow/async/generator/async-gen/non-constructable-builtin/bound-method throw; bound-function + proxy recurse) in js_register_class_parent_dynamic. Also registered async function(){} expression closures as async (they weren't) and made duplicate-named classes re-run the IsConstructor check (test262 reuses class C across blocks). (superclass-arrow/async/generator/async-generator-function)
5. getOwnPropertyDescriptor(C,'prototype') returned undefined. Added the non-writable/non-enum/non-config prototype data-descriptor arm. (prototype-property)
6. call_static_method capped at 4 args — const ref=C.m; ref(a..f) for a static async/gen method dropped args ≥4. Extended to 8 positional params. (*-method-static dflt-params-arg-val-not-undefined)
7. super.x=v in a class with no extends threw "read only". When super is Object.prototype the fallback now ordinary-[[Set]]s on the receiver. (syntax/class-body-method-definition-super-property)

Before → after (per dir, target suite)

• language/statements: 400 → 419 pass; language/expressions: 262 → 266 pass.
• subclass +4, definition +~5, async/gen/async-gen-method-static +~8, syntax +1, gen-method-static +1.

Files

• runtime: closure/{registry,mod}.rs, object/{class_registry,field_set_by_name,descriptors}.rs, proxy.rs
• hir: lower_decl/body_stmt.rs
• codegen: expr/{closure,this_super_call}.rs, lower_call/new.rs, codegen/string_pool.rs, runtime_decls/{objects,stdlib_ffi}.rs

Validation

• Target suite 662 → 685 (+23), 0 regressions.
• --shard 0/12 over built-ins language (2645 judged): 249 rt-fail vs baseline 251 — 0 regressions / 2 fixed.
• Shards 3/12 & 7/12 surfaced 17 apparent diffs; all re-ran clean in isolation (concurrent-shard contention, mostly Temporal).

Deferred (larger separate work)

async-gen yield* return/throw delegation (~34); subclass-of-builtins exotics; derived-ctor this-TDZ / return-override + null-proto; class accessor descriptor callable get/set; symbol-keyed method retrieval; named-class-expression name leaking to outer scope.