Optimize useAnchoredPosition hook for improved INP and rendering performance

.changeset/optimize-anchored-position.md

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+"@primer/react": patch
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+Optimized `useAnchoredPosition` hook for improved rendering performance:
+
+- **Reduced layout thrashing**: Batches DOM reads before writes and uses `requestAnimationFrame` to coalesce multiple update triggers
+- **Optimized re-renders**: Stores mutable state in refs and only triggers re-renders when position actually changes  
+- **ResizeObserver support**: Observes both floating and anchor elements for size changes with fallback to window resize events
+- **Better late-mounting handling**: Properly detects when conditionally-rendered elements mount
+
+The `dependencies` parameter is now deprecated and ignored - position updates are handled automatically via ResizeObserver and window resize events. The parameter is still accepted for backwards compatibility.