[mdjastrzebski]

Stacked on #1. The base is #1's branch, so this diff shows only the new work. Once #1 merges, I'll move the base to main.

Summary

Adds MiniFlatList, a separate component in src/MiniFlatList/ that adds the core ideas of React Native's FlatList on top of NaiveList. NaiveList is unchanged and stays the simplest version.

Prop	Default	What it does
keyExtractor	item.key → item.id → index	Keys the items and their cached sizes, so sizes survive inserts and reorders
extraData	–	Changing it re-renders every item (cells are memoized)
initialNumToRender	10	Items rendered before the viewport is measured. They stay rendered, so scrolling back to the top shows them instantly
windowSize	21	Area kept rendered, in viewports, centered on the visible one
maxToRenderPerBatch	10	Most new items added in one update
updateCellsBatchingPeriod	50 ms	Delay between updates that only fill the area off screen

The defaults match FlatList.

How it works

• Scroll offset in a ref. Scrolling does not re-render the list while the visible items are already rendered. If the visible area is blank, the list renders right away. This mirrors the high- and low-priority update paths of VirtualizedList.
• Batches. computeRenderWindow always renders the visible items, then grows toward windowSize viewports, adding at most maxToRenderPerBatch items that are not already rendered. Items that are already rendered and still inside the target are kept at no cost. After each commit, an effect schedules the next batch. The chain stops once a batch leaves the window unchanged, the same pattern as componentDidUpdate → _scheduleCellsToRenderUpdate.
• Initial items stay mounted. addInitialItems puts the first initialNumToRender items in front of the render window and removes any overlap. The list renders the initial items, a spacer, the window and a tail spacer. This is FlatList's scroll-to-top optimization, which it keeps in the "initial region" of its render mask.
• Memoized cells. A new batch renders only the new items. renderItem must be stable for this to work, as in FlatList.
• Offsets and binary search. computeItemOffsets builds the start offsets once per render. findItemsInArea finds the items in an area with binary search. Unmeasured items count as the average measured size.

The pure logic lives in item-offsets.ts and render-window.ts, and is unit tested apart from React.

Not included (FlatList features for later PRs)

ItemSeparatorComponent, ListHeaderComponent, ListFooterComponent and ListEmptyComponent, onEndReached, getItemLayout, initialScrollIndex and scrollToIndex, onViewableItemsChanged, horizontal, inverted, and clamping the tail spacer to the last measured item.

Other changes

• Fixed the 2 Prettier errors in NaiveList.tsx that make lint fail on feat: add NaiveList, a minimal virtualized list #1. The same fix belongs in feat: add NaiveList, a minimal virtualized list #1, and the two will merge cleanly.
• The example app can switch between NaiveList and MiniFlatList. renderItem is now defined at module level.
• Updated the README.

Testing

• yarn typecheck, yarn lint and yarn test pass: 36 tests in total, 26 of them new.
• Component tests use fake timers to step through batches. They cover initialNumToRender (rendered first and kept after scrolling away), batch growth, the immediate render when the visible area is blank, sizes staying with their keys after a reorder, memoization with extraData, and empty data.
• Not run on a device or simulator yet.