fix(runtime): route for-await async generator values

[andrewtdiz]

Refs #4405

Summary

• adds a ForAwaitToArray HIR materialization path for untyped for await...of receivers
• routes that path through the existing js_array_from_async collector, so async-generator values held in variables or returned from methods use async-iterator/.next() semantics instead of the sync for...of array fallback
• keeps the existing for await array fast path intact, including awaiting array elements
• adds a focused parity fixture covering direct async-generator calls, variable-held async generators, async-generator method results, custom async iterables, and arrays of promises

Why Batched

The variable-held and method-result cases fail for the same reason: after the direct named async-generator call special case is missed, lowering falls through to the untyped for...of materializer. A single async materializer fixes both without changing the direct named-call path.

Non-Goals

• broader async generator scheduling rewrites
• Promise/microtask ordering changes outside the existing Array.fromAsync collector
• adding [Symbol.asyncIterator] shape metadata to Perry async-generator objects
• unrelated for await stream/timer special paths

Validation

• Node 26 oracle for test-parity/node-suite/globals/async-generator-for-await-variable.ts matched Perry direct output
• CARGO_TARGET_DIR=/tmp/perry-async-generator-for-await-check cargo check -p perry-hir -p perry-codegen -p perry-codegen-js passed
• cargo build -p perry-runtime -p perry-stdlib -p perry --release passed
• PATH=/root/.npm/_npx/bac97da9607b7ef2/node_modules/node/bin:$PATH PERRY_NO_AUTO_OPTIMIZE=1 PERRY_NO_CACHE=1 ./run_parity_tests.sh --suite node-suite --module globals --filter async-generator-for-await-variable passed: 1 pass / 0 fail
• same harness with --filter generator passed: 6 pass / 0 fail
• same harness with --filter array-from-async passed: 1 pass / 0 fail
• cargo fmt -p perry-hir -p perry-codegen -p perry-codegen-js -- --check passed
• git diff --check HEAD passed
• git diff --cached --check passed
• jq empty test-parity/known_failures.json passed
• ./scripts/check_file_size.sh passed