Modern UI: side-by-side editor groups should render as separate islands

**Type:** Feature Request
- Setting: `workbench.experimental.modernUI: true`

### Summary

With `workbench.experimental.modernUI` enabled, the workbench elements
(Activity Bar, Side Bars, bottom Panel) are rendered as separate rounded
surfaces with gaps between them. Editor groups are the exception: when the
editor is split, the resulting groups are still drawn as one continuous
surface divided by a 1px sash, exactly as in the classic UI.

### Current behavior

Splitting the editor (e.g. `workbench.action.splitEditorRight`) produces two
groups that share a single rounded container. The split line between them is
the old thin divider, so the boundary between the two groups is much weaker
than the boundary between, say, the Side Bar and the editor area. Visually the
two groups read as one region that happens to have a line through it.

### Expected behavior

Each editor group should be its own island: its own rounded corners, its own
border, and the same gap that separates the other workbench surfaces. This
should apply to:

- horizontal and vertical splits
- nested / grid layouts (`workbench.action.editorLayoutTwoByTwo` etc.)
- groups created by dragging a tab to an edge
- the two sides of a side-by-side diff editor (or an explicit note if this one
is intentionally excluded, since the two panes there are one logical editor)

The outer editor area should keep its existing gap to the Side Bar and Panel,
so nothing shifts when going from one group to two.

### Screenshot