Create meaningful Maestro scroll anchor test

[dnorman]

Context

Had a Maestro test (05_scroll_anchor_pagination.yaml) that was supposed to verify scroll anchor behavior during pagination. However, the test was ineffective because:

1. All messages were identical ("Test message for scroll" x30)
2. No way to verify a specific message stayed anchored in the viewport
3. Screenshots were taken but not compared programmatically
4. Essentially just a smoke test that other tests already cover

The test was deleted.

Desired behavior

A meaningful scroll anchor test should:

1. Use distinguishable messages - numbered or timestamped so position can be verified
2. Verify anchor stability - when scrolling triggers pagination (loading more messages), the currently visible message should stay in the same viewport position
3. Detect jumps - if the scroll position jumps unexpectedly, the test should fail

Possible approaches

1. Screenshot comparison - compare pixel positions of a known element before/after pagination
2. Accessibility label verification - check that a specific message ID is still at the same relative position
3. Custom test harness - expose scroll position metrics via testID and verify they remain stable