fix(hir): class test262 remnant (elements/dstr/private/async tail)

[proggeramlug]

Summary

Class-suite test262 tail (language/statements/class + language/expressions/class): 4839 → 5009 pass (90.1% → 93.3%, +170, zero regressions). Broad cross-suite shard (--shard 0/12 built-ins language): 91.5% → 92.0% (+15 fixed, zero regressions). Native-region gate: pass (7/7 workloads). cargo test (hir/codegen/transform/runtime): all pass.

Root causes fixed

1. PerformEval early errors for eval in class contexts (~120 tests; biggest bucket). New crates/perry-hir/src/lower/eval_super_scan.rs: constant direct-eval bodies are scanned per spec Contains semantics (arrows transparent; function / class-member / object-method bodies opaque) for SuperCall, SuperProperty, arguments, new.target, and private-name references. Capabilities come from the lowering context: super() allowed only directly in a derived-class ctor (new current_class_is_derived), super.x in any class member / object method, arguments rejected in field initializers (new in_class_field_init, cleared inside nested plain functions), private names checked against the lexical private_scopes stack. Violations compile to a runtime SyntaxError via new js_throw_eval_syntax_error. Indirect (0, eval) evaluates as global code: any super or new.target rejected; private names valid only if declared inside the eval source.

2. Direct-eval wrapper rebound this (~30 tests). The constant-eval scope-IIFE template was a plain function, so eval(\"this.#x\") inside a method brand-checked against undefined and eval(\"this.prop\") read undefined. The wrapper is now an arrow, giving eval'd code the caller's this / arguments / super / new.target lexically (spec: direct eval inherits all four).

3. Allowed super.x in eval bodies failed to parse (~24 tests). SWC rejects SuperProperty at script top level, so x = eval('() => super.x') threw a bogus SyntaxError. When the call site provides super, the body re-parses inside an object-method wrapper and the method body statements are spliced into the (now-arrow) IIFE.

4. Derived-ctor static no-super throw too eager (~10 tests). The compile-time ReferenceError: must call super now stays silent when the ctor body has (a) a value-bearing return (return-override path; bare return undefined still throws — Object/constructor-return-undefined-throws), or (b) a closure-captured super() with no direct this use (e.g. iter.f = () => super() invoked during for-of iterator close — derived-class-return-override-{for-of,finally-super,catch-finally}-arrow; the direct-this carve-out keeps privatefieldset-evaluation-order-1 throwing).

5. arguments in parameter defaults (8 tests). method(x = arguments[2], y) {} never synthesized the arguments object (the check only scanned the body), and the synth param was appended after defaults were lowered, so arguments in a default resolved as an unknown global. Both fixed for class methods and free functions (class_members.rs, fn_decl.rs).

6. this in static field initializers (~4 tests). static g = this.f + '262' read undefined — init_static_fields_late now seeds the same class-ref NaN-box this slot a static method binds.

Validation

• 4 incremental box sweeps; failure-set diffs at each step (not aggregate %); the two intermediate regressions introduced by fix 4 (privatefieldset-evaluation-order-1, constructor-return-undefined-throws) were caught by the set-diff and fixed (direct-this carve-out + return undefined exclusion) — final diff vs merge-base f32b486 is fixes-only, in-suite and cross-suite.
• Native-region gate: compiler_output_regression.py suite --suite native-region-proof → failed_workloads: [].
• cargo fmt --all applied; file-size gate OK (largest touched file 1706 lines).

Knowingly deferred (tracked)

• async-gen yield* return/throw forwarding (~80 remaining failures): the async-generator .throw()/.return() closures use deferred-resume and cannot re-enter the state machine; forwarding to the delegated iterator needs that contract rebuilt (pre-existing deferral from fix: yield* async-delegation test262 parity (next-capture + object-literal thenable await) #4783/fix(runtime): class subclass + async-method + definition test262 tail #4853).
• Static private members accessed via C.m.call(arbitraryObj) (~12): static-method dynamic this hardcodes the class ref (deferred in fix(hir): class field/element initializer test262 tail (v3, native-region-safe) #4847).
• extends null semantics, subclass-builtins exotics, per-field private install-order brand checks, named-class inner-binding immutability: each small, distinct, pre-existing buckets.

No version bump / changelog edits per external-contributor flow.