Clean up IndexedDB previous-page listener

[issackjohn]

Follow-up for local issue 52.

This replaces the hidden promiseResolve state stored on iterationFinishedListener with a local one-shot previous-page-loaded promise for each page transition.

The listener is now registered immediately before clicking the previous-page button, so the event cannot be missed, and it is automatically removed after firing.

Validation:

• git diff --check

Note: local Node/npm tooling was not available in this shell, so browser CI should cover the end-to-end validation.

[Copilot]

Pull request overview

This PR cleans up the IndexedDB TodoMVC benchmark’s “previous page loaded” synchronization by replacing a shared, hidden promiseResolve function property with a per-transition one-shot promise tied to the previous-page-loaded event. This makes page-transition waiting deterministic (listener installed immediately before the click) and ensures the listener is automatically removed after firing.

Changes:

• Replace the persistent previous-page-loaded listener + iterationFinishedListener.promiseResolve state with waitForPreviousPageLoaded() that uses addEventListener(..., { once: true }).
• Register the listener immediately before clicking the previous-page button, then await the resulting promise.
• Apply the same update to both the source and generated dist workload test files.

Reviewed changes

Copilot reviewed 1 out of 2 changed files in this pull request and generated no comments.

File	Description
experimental/javascript-wc-indexeddb/src/workload-test.mjs	Switches “previous page loaded” waiting logic to a per-iteration one-shot promise to avoid missed events and hidden listener state.
experimental/javascript-wc-indexeddb/dist/src/workload-test.mjs	Keeps generated dist workload test in sync with the updated source behavior.

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