Clean up IndexedDB previous-page listener state

[issackjohn]

Follow-up from WebKit#531 review.

Review context:

• Reviewer comment: github dot com/Add TodoMVC with indexedDB storage WebKit/Speedometer#531#discussion_r3188843379
• My follow-up promise: github dot com/Add TodoMVC with indexedDB storage WebKit/Speedometer#531#discussion_r3204794094

Current state:

• experimental/javascript-wc-indexeddb/src/workload-test.mjs and the checked-in dist/src/workload-test.mjs define iterationFinishedListener() inside DeletingAllItems.
• The listener stores the active resolver as a property on the function object:

function iterationFinishedListener() {
    iterationFinishedListener.promiseResolve();
}
window.addEventListener("previous-page-loaded", iterationFinishedListener);
...
const iterationFinishedPromise = new Promise((resolve) => {
    iterationFinishedListener.promiseResolve = resolve;
});

Why this is worth cleaning up:

• It works, but the resolver is hidden mutable state on a function object.
• The listener is registered once and never removed.
• The pattern is surprising for future benchmark maintenance.

Suggested direction:

• Replace the function-property resolver with a local promise/event helper, e.g. waitForPreviousPageLoaded() that registers a one-shot listener before clicking the previous-page button.
• Keep the listener registration before the click, so the event cannot be missed.
• Update both source and generated/dist copies consistently if the workload still keeps checked-in dist sources.

Validation:

• Run the IndexedDB workload in the browser matrix, or at least npm run test-e2e:safari / npm run test-e2e:chrome if available locally.