Grid Multi-Body: Adapt Keyboard Navigation for Split Bodies

As part of the Multi-Body Grid architecture Epic (#9486), Keyboard Navigation must be updated to treat the split physical SubGrids as a single continuous logical grid.

The Challenge: Currently, using Left/Right arrow keys navigates through the DOM structure of a single `grid.Body`. In the multi-body setup, the `grid.Body` is split.

Requirements:

1. Logical Coordinate System: KeyNav must operate entirely on logical coordinates (Column Index x Record Index) rather than physical DOM siblings.
2. Boundary Crossing: Navigating Right from the last cell in the `locked: 'start'` body must logically move focus/selection into the first cell of the `center` body.
3. Cross-Window Support: This KeyNav logic must work even if the `start` body has been ripped out into a separate physical browser window (as outlined in #9493), by updating the shared `SelectionModel` state in the App Worker.