Replaces scroll_fraction-only position tracking with element-based
anchors ("{index}:{innerFraction}"). Position is now stable across
font size changes and different screen sizes. A ResizeObserver
restores the anchor on viewport/orientation changes.
Falls back to scroll_fraction for books without a saved anchor.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
