Data Grid Status Bar

Status-bar add-on for the gridcn data grid: <DataGridStatusBar /> shows count, sum, avg, min, and max of the selected cells under the grid (useDataGridSelectionStats is the headless hook). Numbers are detected by value (finite numbers), sums use Neumaier-compensated summation, a single-column selection formats through that column's toDisplayText, and above 200,000 cells the walk caps and flags the values with ≈. New user-facing strings live under labels.statusBar (see data-grid/labels.ts DataGridStatusBarLabels). Zero core changes — purely additive.