test: wait for settled reads in the unread filter walk test

[xsmyile]

The setsize check now retries until the in-flight read events land; the total always settles, so this is a test race, not a UI bug.

Closes #130

[pullfrog]

✅ No new issues found.

Reviewed changes Reviewed the unread-filter forward-walk test adjustment and its asynchronous read-event handling.

• Retry the exact count assertion. aria-setsize now uses an awaited toHaveAttribute assertion so pending read events can settle, while preserving the exact expected count and existing navigation checks.
• Verification. The affected test passed 10 repetitions with four Chromium workers; the test server also completed TypeScript checking and the production frontend build.

  ^{｜ View workflow run ｜ Using gpt-6.1-sol ｜ 𝕏}