runtime: settle async results before suspended producer exit

[lannbot]

Summary

Fixes #323.

Allow an async export driver to finish once its result is captured and entry hops are clear, even while its producer remains genuinely JSPI-suspended. The existing settlement pump owns continuation and late failures. Sync-typed lifts retain activation-completion requirements.

Adds real-Wasm coverage for result delivery before gate release, task.return after an earlier suspension, a nonblocking JSPI hop before another suspension, continued producer progress, and retained guest/host failures. A store-level test checks that unrelated failures do not unwind the live producer.

Ordering sanity check

Inspected Wasmtime at the repository's pinned revision 4675ee1: task_return/task_complete send the host result immediately (concurrent.rs:3507–3557,3606–3608), and run_concurrent polls the caller future before background futures (1320–1328). This corroborates separate result and producer lifetimes; the Component Model reference and embedder contract remain the authorities. This was a source-level check, not a Wasmtime differential execution.

Validation

• just gates < /dev/null passed, including Chromium/Firefox page, worker, shared-worker lanes and consumer smokes; independently repeated by review.
• Focused runtime tests: 74 passed.
• Four new tests passed with tracing enabled.
• just fmt-check, git diff --check, and Wasm fixture validation passed.
• Independent review: LGTM.

Automerge will be armed after opening this PR.