lift: settle an idle-exited async export on task.return, not thread drain

[lannbot]

Fixes #313.

backgroundCompletion (#310) resolved an idle-exited async lift on Task.onFinished — the task's last thread unregistering. A callback-ABI export that keeps spawned futures alive for the instance's life (wit-bindgen spawn_local: event loop, driver, accept loop) never reaches that moment, so a lift that went idle before task.return and was later woken by another driver had its results captured and its host Promise left pending forever. Silent hang, found while tracing #312.

The reference's answer event is on_resolve, called from Task.return_ (definitions.py); lift_and_run keeps ticking afterwards for other work, not to produce the result. wasmtime's call_concurrent has the same shape. The lift's own onResolve closure now fires a one-shot hook that settles the Promise; an already-resolved task settles at once (generalizing the old threads.length === 0 case). No deferral to "no wasm call in flight": driveDone's midWasmCall/hopParked clauses keep a driver driving under a suspended activation, and on this path there is no lift driver left to stop.

Task.onFinished and its unregisterThread firing block are deleted — this obsoletes them and nothing else used them.

Test: runtime/tests/lift_background_return_test.ts, store-level in the style of lift_done_verdict_test.ts. An export goes idle with an immortal thread and a returner parked on a flag; a later export call drives the store; the returner calls task.return; the first Promise resolves while the immortal thread is still parked (asserted, so drain is not what settled it). Red on main ("expected {v:undefined}, got pending"), green with the fix; also green under POLYENGINE_SCHED_SEED=1 and 7.

Gates: just test-runtime — 739 passed, 0 failed. Goldens untouched; no breaking/* label (bug fix).

Automerge armed.