Review IndexedDB workload DB synchronization boundaries

[issackjohn]

Follow-up from WebKit#531 review.

Review context:

• Reviewer comment: github dot com/Add TodoMVC with indexedDB storage WebKit/Speedometer#531#discussion_r3189036408
• My follow-up promise: github dot com/Add TodoMVC with indexedDB storage WebKit/Speedometer#531#discussion_r3204793935

Current state:

• experimental/javascript-wc-indexeddb/src/workload-test.mjs splits DB completion into ignored follow-up steps:
  • FinishAddingItemsToDB waits for db-add-completed.
  • FinishModifyingItemsInDB waits for db-toggle-completed.
  • FinishDeletingItemsFromDB waits for db-delete-completed.
• The storage layer dispatches those events from src/storage/base-storage-manager.js after all expected DB operations for that phase complete.
• DeletingAllItems also pages backward via previous-page-loaded, which is currently a UI/page-load synchronization signal, not a DB synchronization signal.

Open question from review:

• Are delete/read/page phases synchronized enough to compare performance reliably across browsers and storage backends?
• In particular, deleting visible items and then paging/readback may need clearer DB synchronization boundaries if reads are involved in loading the previous page.

Research notes:

• TodoApp.moveToPreviousPage() awaits this.list.moveToPreviousPage() before dispatching previous-page-loaded.
• The benchmark separately waits for db-delete-completed only after all delete/page iterations finish.
• Need to verify whether each page transition reads from IndexedDB and whether those reads are included in the intended measured phase.

Suggested direction:

• Trace todo-list.moveToPreviousPage() and storage calls for IndexedDB and DexieJS.
• Decide whether page-load/read work should be measured in DeletingAllItems or isolated in an ignored sync step.
• If needed, add explicit per-page DB read completion events or await existing storage promises instead of relying only on previous-page-loaded.
• Document the intended measurement boundary in workload-test.mjs so future changes keep IndexedDB and DexieJS comparable.