Function pointToGridCell

Compute the integer cell {col, row} that contains point, given a grid spacing and optional origin and unitSystem. Pair with createCellHighlightLayer (its getCell callback) to draw a snap-target preview that uses the same spacing as gridSnapStrategy — so the visual and behavioral grids stay in lockstep:

const SPACING = 20;
const snap = gridSnapStrategy(SPACING);
// visual grid:
createGridLayer({ spacing: SPACING, bounds: () => ... });
// hover-preview overlay:
createCellHighlightLayer({
  spacing: SPACING,
  getCell: () => hoverPoint && pointToGridCell(hoverPoint, SPACING),
});