Diagram tab: Structure tool window integration

## Context

The Graph tab exposed `getStructureViewBuilder()` via `GraphStructureViewBuilderSetup`, so the Structure tool window could reflect the graph hierarchy.

The Diagram tab does not implement structure view integration.

## Proposal

Provide a Structure tool window tree when the Diagram tab is active.

## Acceptance criteria

- [ ] Structure tool window lists packages → actions → results (or equivalent tree) when Diagram tab is active
- [ ] Selecting a structure node highlights the diagram node
- [ ] Selection syncs with the XML perspective where applicable

## References

- `Struts2GraphFileEditor.getStructureViewBuilder()`
- `Struts2DiagramFileEditor` — no structure view builder today

Compare Struts2GraphFileEditor.getStructureViewBuilder() and GraphStructureViewBuilderSetup with Struts2DiagramFileEditor, which currently has no structure view builder. Trace how the existing Graph tab exposes packages, actions, and results, then verify the Diagram tab’s Structure tool window, diagram selection, and XML perspective synchronization against the acceptance criteria.

Written by the indexing model from the issue text.