[andrewtdiz]

Summary

• wrap lowered async generator next / return / throw methods with a runtime request queue
• drain same-stack async generator requests through microtasks so ordering matches Node
• add internal Promise settle listeners and scanner roots so queue bookkeeping does not consume user-visible .then slots
• add a Node parity fixture for same-stack async generator request ordering

Root Cause

Lowered async generator instance methods could be invoked reentrantly without Node-compatible request queue semantics. That let same-stack next() calls race the in-flight request instead of being serialized behind it.

Fixes #4438

Validation

• cargo check -q -p perry-runtime --lib
• cargo build --release -p perry-runtime -p perry with CARGO_TARGET_DIR=/root/perry-worktrees/perry-node-probe-20260604/target CARGO_BUILD_JOBS=1
• cargo fmt --all -- --check
• git diff --check and git diff --cached --check
• ./scripts/check_file_size.sh
• cargo test -q -p perry-runtime test_gc_init_mutable_scanner_families_rewrite_runtime_slots with shared CARGO_TARGET_DIR
• direct Node 26 vs Perry diffs for async-generator-request-queue.ts, generator intrinsic/prototype fixtures, stream/readable/from-async-iterable.ts, and Promise queue fixtures