Diagram tab: sync selection with XML perspective

## Context

The Graph tab supported **graph selection → DOM** via `getSelectedDomElements()`, feeding the shared `PerspectiveFileEditor` selection.

The Diagram tab returns `null` / no-op for both `getSelectedDomElement()` and `setSelectedDomElement()`.

Note: Graph's reverse sync (XML → graph highlight) was stubbed with a TODO and never worked — this is an opportunity to implement both directions properly.

## Proposal

Bidirectional selection sync between the Diagram canvas and the XML perspective editor.

## Acceptance criteria

- [ ] Clicking a diagram node selects/highlights the corresponding DOM element in the XML perspective
- [ ] Selecting an action/result/package in the XML tree highlights the matching diagram node (when visible)
- [ ] Single-click selects; double-click continues to navigate (current behavior)
- [ ] Works for package, action, and result nodes

## References

- `Struts2DiagramFileEditor.getSelectedDomElement()` / `setSelectedDomElement()` — currently no-ops
- `Struts2GraphFileEditor` — had partial graph → DOM sync

Start with Struts2DiagramFileEditor.getSelectedDomElement() and setSelectedDomElement(), then compare their no-op behavior with Struts2GraphFileEditor and the shared PerspectiveFileEditor selection. Trace the existing single- and double-click handling and XML-tree selection flow. Done means package, action, and result selections synchronize in both directions, while double-click navigation remains unchanged.

Written by the indexing model from the issue text.