116Implement a virtualized grid with variable row heights, overscan, and scroll restoration

Build a grid that can show 100,000 rows × 50 columns, where rows have different heights.

First, pure helpers:

• buildOffsets(rowCount, getRowHeight): an array offsets of length rowCount + 1, where offsets[i] is the top of row i and offsets[rowCount] is the total height.
• findRow(offsets, y): the index of the row containing pixel y, using binary search, clamped to valid rows.
• getRange(offsets, scrollTop, viewportHeight, overscan): { start, end } (inclusive) of the rows to render, including overscan extra rows on each side.

Then <VirtualGrid rowCount columnCount getRowHeight columnWidth width height overscan={2} renderCell initialScrollTop={0} onScroll />:

• a scroll container data-testid="viewport" of width × height, with a spacer sized to the full grid;
• render only the visible rows and the visible columns (plus overscan), each cell absolutely positioned with data-row and data-col, containing renderCell(row, col);
• on mount, restore initialScrollTop (before the first paint), and call onScroll({ scrollTop, scrollLeft }) on every scroll so the position can be saved (for example in sessionStorage).