Date picker: month navigation feels laggy on low-end devices

Navigating between months on my old Android phone takes almost a second, the animation stutters badly.— user feedback, support ticket #482

Reproduced on a throttled 4x CPU profile. Each month navigation re-renders 42 day cells plus the header, and every cell recomputes its formatter.

Hypotheses

• The Intl.DateTimeFormat instance is created per cell per render — hoist and reuse
• Month transition animates box-shadow (paint-heavy), switch to transform/opacity
• Day cells can be memoized: only selection and today change between renders