Apply offset(y: -stretchAmount) to gradient, buttons, summaryCard, storeLogoBadge, and hard-cut rectangle so only the cover image stretches on pull-down
- Remove max(0) clamp in ScrollOffsetReader to allow negative values during bounce
- Add stretchAmount computed var (max(0, -scrollOffset))
- Hero image frame grows by stretchAmount and offsets upward on overscroll
- isCategoryTabsPinned still uses max(0, scrollOffset) via >= check
