Clean up IndexedDB previous-page listener

[issackjohn]

Summary

Rework how the DeletingAllItems step waits for the IndexedDB TodoMVC app to finish a
previous-page navigation.

Problem

The step kept its promise resolver as a property hung off the iterationFinishedListener
function object, and registered a previous-page-loaded listener that was never removed:

• The resolver was shared mutable state on a function, so which promise a given event
  resolved depended on loop ordering rather than being explicit.
• The listener outlived the step, since nothing ever called removeEventListener.
• A promise was allocated on every one of the 10 iterations, even though the last iteration
  never navigates and never awaits it.

Change

Add a module-scope waitForPreviousPageLoaded() helper that returns a promise backed by a
{ once: true } listener, and call it immediately before clicking the Previous button.
Each page transition now owns its own listener and resolver, and the listener removes itself
once the event fires.

previous-page-loaded is dispatched only from todo-app's moveToPreviousPage(), which
runs only in response to the bottombar's previous-page event from that button click. So
registering the listener just before the click cannot miss the event or observe a stray one.

This is a refactor of the benchmark's synchronization only. The measured work is unchanged.

Notes

dist/src/workload-test.mjs is regenerated from src/workload-test.mjs by
npm run build in suites-experimental/javascript-wc-indexeddb; both are updated here so
the checked-in build stays in sync.

Validation

• npm run test:node — 66 passing, including the resources.txt validity check
• eslint and prettier --check clean on the changed source

[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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