runtime/codegen: async function semantic parity

[andrewtdiz]

Summary

Track async function completion, default parameter, this/arguments/new.target, and early-error parity gaps without conflating them with Node stdlib async APIs.

Child issues

• runtime/codegen: resolve async function completion, return, throw, and await timing #3566
• runtime/codegen: evaluate async function default parameters and trailing-comma length semantics #3567
• runtime/codegen: preserve this, arguments, caller/callee, and new.target capture in async functions #3568
• parser/hir: handle async function declaration completion and early-error cases #3569

Scope notes

• Treat these as Perry ECMAScript/runtime-language semantics gaps, not Node stdlib parity.
• Likely subsystems are parser, HIR lowering, codegen, runtime object model, async scheduling, or error/throw semantics.
• Eval/runtime-dynamic-code rows are excluded separately under Perry's AOT policy.

Validation

scripts/test262_subset.py --root vendor/test262 --dir language/statements/async-function --sample-cap 99999 --report /tmp/perry-test262-async-function.json

Acceptance for this epic: every linked child issue has a focused regression or Test262 validation path, and the affected rows are removed from the gap report without introducing duplicate issue coverage.

[andrewtdiz]

The async-function child issue set now appears complete:

• runtime/codegen: resolve async function completion, return, throw, and await timing #3566 is closed.
• runtime/codegen: evaluate async function default parameters and trailing-comma length semantics #3567 is closed.
• parser/hir: handle async function declaration completion and early-error cases #3569 is closed.
• runtime/codegen: preserve this, arguments, caller/callee, and new.target capture in async functions #3568 has closure-ready coverage from merged PR [codex] Preserve lexical new.target for arrows #4224 (new.target capture) and merged PR [codex] Honor function accessor properties #4243 (caller accessor tail), with earlier current-main probes showing sampled async this and mapped arguments rows matching Node.

This epic looks ready for maintainer closure once #3568 is marked complete.