[focusgroupV2] Define coordinate topology for irregular grids

The initial Focusgroup V2 grid model is rectangular and spanless. Unequal row lengths and effective rowspan or colspan values greater than one invalidate the grid.

<table role="grid" focusgroup="grid" aria-label="Templates">
  <tr>
    <td tabindex="0">Blank</td>
    <td tabindex="0">Report</td>
    <td tabindex="0">Invoice</td>
  </tr>
  <tr>
    <td tabindex="0">Add template…</td>
  </tr>
</table>

This keeps the initial model deterministic, but excludes partial rows, merged cells, and sparse grids.

One possible extension is to preserve gaps as implicit empty coordinates. These coordinates would participate in topology and column calculations without becoming focus destinations. This differs from collapsing or skipping the missing positions when assigning coordinates.

We need to define:

1. Whether gaps remain implicit coordinates or are removed from the coordinate space.
2. Whether movement encountering an empty coordinate stops or searches farther in the requested direction.
3. How wrap and flow interact with empty coordinates.
4. How spans occupy coordinates and affect block-axis column preservation.
5. Which coordinate represents a spanning cell for memory and focusgroupstart.

Authored ARIA should continue to describe accessibility semantics rather than configure focus topology.

Related:

• Focusgroup V2 explainer changes
• Focusgroup V2 topology
• PR #1494 review discussion
• #1390: Multi-directional focus groups
• APG Grid and Table Properties

Start with the Focusgroup V2 topology explainer and the review discussion on PR #1494, then compare the APG Grid and Table Properties guidance. Define decisions for gaps, movement, wrapping, flow, spans, and focus memory; done means the topology behavior is specified consistently.