Pure layout algorithm: given a list of sidebar items + their anchor sources, return a list of resolved Y positions with collision avoidance applied. Lifted from React's components/sidebar/resolveItemPositions.ts so the Vue adapter can call it directly.

The algorithm walks the items in three priority stages: 1. explicit fixedY (already in scroll-container coords) 2. anchorKey lookup in anchorPositions (layout-engine coords) 3. DOM-based anchor lookup via renderedDomContext.getRectsForRange 4. last-known cache so cards don't pop out during transient layout

Then it sorts by target Y and pushes overlapping cards down by their height + MIN_CARD_GAP.