Function pickHex

pickHex(
    clientX: number,
    clientY: number,
    domElement: HTMLElement,
    camera: Camera,
    layout: HexLayout,
    map: { width: number; height: number },
    planeY?: number,
): { col: number; row: number } | null
Picks a hex cell by raycasting against a horizontal plane at planeY (default 0).

Faster than pickHexFromMeshes but drifts at low camera angles over elevated terrain. Useful for flat maps or water surfaces where geometry accuracy isn't critical.

Returns null if the ray misses the plane or the hit is outside the map bounds.
Parameters
- clientX: number
- clientY: number
- domElement: HTMLElement
- camera: Camera
- layout: HexLayout
- map: { width: number; height: number }
- planeY: number = 0
Returns { col: number; row: number } | null
Example
```
canvas.addEventListener('pointermove', e => {
  const cell = pickHex(e.clientX, e.clientY, renderer.domElement, camera, layout, map);
  if (cell) highlight(cell.col, cell.row);
});
```
- Defined in geometry/HexPicking.ts:72