Feature request: Support shared resizing across two-dimensional grid layouts

## Feature request: Support shared resizing across two-dimensional grid layouts

### Description

I am trying to build a resizable two-dimensional grid using `react-resizable-panels`.

For example:

```text
┌───────────┬───────────┐
│ A │ B │
├───────────┼───────────┤
│ C │ D │
└───────────┴───────────┘
```

The expected behaviour is:

* Dragging the vertical separator should resize both columns across every row.
* Dragging the horizontal separator should resize both rows across every column.
* Optionally, dragging the centre intersection could resize both axes.

### Current behaviour

The closest implementation is to nest panel groups:

```tsx

A

B

C

D

```

However, the horizontal groups are independent.

Dragging the separator between `A` and `B` does not move the separator between `C` and `D`.

Reversing the nesting solves the shared column resizing, but then the row separators become independent.

This means the layout can only have shared resizing along one axis at a time.

### Requested capability

It would be useful to support grid-style shared tracks, where multiple panels can reference the same row or column size.

A possible API could look similar to:

```tsx

A
B
C
D

```

Alternatively, nested groups could expose a supported way to synchronise their layouts without manually coordinating several refs and `setLayout()` calls.

Start by reviewing the existing Group, Panel, and Separator APIs, then examine how nested layouts are managed. Compare the requested shared row and column behavior with the current ref/setLayout() synchronization approach mentioned in the issue. Done means a supported grid or synchronization capability lets separators resize matching tracks across all rows or columns.

Written by the indexing model from the issue text.